csdl tonghop-sua2

8/8/2019 CSDL Tonghop-sua2

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U BAN NHN DN THNH PH H NI

S BU CHNH VIN THNG THNH PH

GIO TRNHC S D LIU

(M s gio trnh: 2CD3)

H NI 2005

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Li m u

Ngy nay, c s d liu c nhiu ng dng trong mi hot ng ca xhi. Mun thit k v s dng c s d liu chng ta phi nm c cc k thutc bn ca c s d liu. Gio trnh ny nhm trnh by cc k thut c s ca c

s d liu truyn thng, l m hnh lin kt thc th, m hnh c s d liu quanh. Gio trnh cng trnh by cch thit k mt c s d liu quan h, cch s dngcc php ton i s quan h to, cp nht v truy vn c s d liu v khinim ph thuc hm ng dng trong l thuyt thit k v chun ha c s d liuquan h.

Gio trnh cn thit cho tt c cc i tng mun tm hiu v thit k cc cs d liu quan h ng dng trong cng tc qun l.

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- Tn mn hc: C s d liu.

- M s mn hc: 2CD3.

- Thi gian: L thuyt + Bi tp 45 tit.

- Mc tiu: Trang b cc kin thc c bn v C s d liu truyn thng.

- Nhng kin thc cn phi c trang b trc khi hc: khng.

- Ni dung:

Chng I: CC KHI NIM C BN V C S D LIU

Chng II: M HNH LIN KT THC THChng III: M HNH QUAN H, CC RNG BUC QUAN

H V I S QUAN H

Chng IV: PH THUC HM V CHUN HA CSDL

QUAN H, CC THUT TON THIT K CSDL

QUAN H

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MC LCCHNG I- CC KHI NIM C BN V H C S

D LIU.........................................................................................................................................................................8

I- C s d liu.................................................................................................................8 I.1- nh ngha c s d liu....................................................................................................................8I.2- Cc tnh cht ca mt c s d liu...................................................................................................9

II- H qun tr c s d liu..........................................................................................10II.1- nh ngha h qun tr c s d liu..............................................................................................10II.2- Cc chc nng ca mt h qun tr c s d liu..........................................................................10II.3- Cc c trng ca gii php c s d liu.....................................................................................12II.4- V d v mt c s d liu...............................................................................................................14

III- M hnh c s d liu .............................................................................................16III.1- Cc loi m hnh c s d liu......................................................................................................16

III.2- Lc v trng thi c s d liu...............................................................................................18IV- Con ngi trong h c s d liu ..........................................................................19IV.1- Ngi qun tr h c s d liu (Database Administrator DBA)...............................................19IV.2- Ngi thit k c s d liu (Database Designer)........................................................................19IV.3- Nhng ngi s dng (End User)..................................................................................................19IV.4- Ngi phn tch h thng v lp trnh ng dng ............. ............... .............. .............. ............ .....20IV.5- Ngi thit k v ci t h qun tr d liu.................................................................................20

IV.6- Nhng ngi pht trin cng c ...................................................................................................20IV.7- Cc thao tc vin v nhng ngi bo tr.....................................................................................20

V- Ngn ng c s d liu v giao din........................................................................21V.1- Cc ngn ng h qun tr c s d liu..........................................................................................21

V.2- Cc loi giao din h qun tr c s d liu...................................................................................22VI- Cu hi n tp..........................................................................................................22

CHNG II- M HNH THC TH - LIN KT....................................................................................24

I- S dng m hnh quan nim bc cao cho vic thit k c s d liu.....................24

II- Cc thnh phn c bn ca m hnh ER................................................................26 II.1- Thc th v thuc tnh.....................................................................................................................26II.2- Kiu thc th, tp thc th, kha v tp gi tr...............................................................................28II.3- Kiu lin kt, tp lin kt v cc th hin........................................................................................30II.4- Cp lin kt, tn vai tr v kiu lin kt quy..............................................................................32

II.5- Cc rng buc trn cc kiu lin kt ............... .............. .............. .............. .............. ............... .......32II.6- Thuc tnh ca cc kiu lin kt......................................................................................................34 II.7- Cc kiu thc th yu .....................................................................................................................34

III- V d v thit k m hnh ER.................................................................................35III.1- Xc nh cc kiu thc th, cc thuc tnh v cc kiu lin kt.....................................................36

IV- M hnh thc th lin kt m rng (m hnh EER).............................................39IV.1- Lp cha, lp con v s tha k......................................................................................................39 IV.2- Chuyn bit ho, tng qut ho.....................................................................................................40

IV.2.1- Chuyn bit ho....................................................................................................................................40

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IV.2.2- Tng qut ho.......................................................................................................................................42IV.2.3- Phn cp chuyn bit v li chuyn bit.............................................................................................43IV.2.4- Cc rng buc v cc c trung ca chuyn bit ho, tng qut ho.....................................................43

IV.3- S m hnh EER........................................................................................................................44

V- Tng kt chng v cu hi n tp.........................................................................45V.1- Tng kt chng..............................................................................................................................45

V.2- Cu hi n tp.................................................................................................................................45V.3- Bi tp.............................................................................................................................................46

CHNG III- M HNH QUAN H, CC RNG BUC QUAN H V I S QUAN H...... .....48

I- Cc khi nim ca m hnh quan h.........................................................................48 I.1- Min, thuc tnh, b v quan h.......................................................................................................48I.2- Cc c trng ca cc quan h........................................................................................................50

I.2.1- Th t ca cc b trong mt quan h .....................................................................................................50I.2.2- Th t ca cc gi tr bn trong mt b ..................................................................................................51I.2.3- Cc gi tr trong mt b...........................................................................................................................51I.2.4- Th hin ca mt quan h.......................................................................................................................52

II- Cc rng buc quan h, lc c s d liu quan h.........................................52 II.1- Cc rng buc min........................................................................................................................52

II.2- Rng buc kho v rng buc trn cc gi tr khng xc nh (null)............................................53II.3- C s d liu quan h v lc c s d liu quan h................................................................54II.4- Ton vn thc th, ton vn tham chiu v kho ngoi..................................................................58

III- Cc php ton trn m hnh quan h....................................................................60 III.1- Cc php ton cp nht.................................................................................................................60

III.1.1- Php chn (Insert).................................................................................................................................60III.1.2- Php xo (Delete).................................................................................................................................61III.1.3- Php sa i (Update)..........................................................................................................................62

III.2- Cc php ton i s quan h........................................................................................................62III.2.1- Php chn (SELECT)...........................................................................................................................63

III.2.2- Php chiu (PROJECT)........................................................................................................................65III.2.3- Php t li tn (RENAME).................................................................................................................66III.2.4- Cc php ton l thuyt tp hp............................................................................................................67III.2.5- Php ni (JOIN)....................................................................................................................................70III.2.6- Tp hp y cc php ton quan h.................................................................................................72III.2.7- Php chia..............................................................................................................................................73

III.3- Cc php ton quan h b sung.....................................................................................................74III.3.1- Cc hm nhm v cc php nhm........................................................................................................74III.3.2- Cc php ton khp kn quy.............................................................................................................75III.3.3- Cc php ton ni ngoi (outer join), hp ngoi (outer union)..............................................................75

III.4- Mt s v d v truy vn trong i s quan h...............................................................................76

IV- Chuyn i m hnh ER thnh m hnh quan h.................................................77 IV.1- Cc quy tc chuyn i..................................................................................................................77IV.2- Chuyn i m hnh c th ............................................................................................................81

V- Tng kt chng v cu hi n tp..........................................................................81V.1- Tng kt chng..............................................................................................................................81V.2- Cu hi n tp.................................................................................................................................82V.3- Bi tp.............................................................................................................................................83

CHNG IV- PH THUC HM V CHUN HA C S D LIU QUAN H, CC THUTTON THIT K C S D LIU QUAN H.....................................................................................................86

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I- Cc nguyn tc thit k lc quan h..................................................................86I.1- Ng ngha ca cc thuc tnh quan h.............................................................................................86I.2- Thng tin d tha trong cc b v s d thng cp nht...............................................................87

I.3- Cc gi tr khng xc nh trong cc b..........................................................................................89 I.4- Sinh ra cc b gi.............................................................................................................................89

II- Cc ph thuc hm...................................................................................................90

II.1- nh ngha ph thuc hm..............................................................................................................90II.2- Cc quy tc suy din i vi cc ph thuc hm............................................................................93

II.3- S tng ng ca cc tp ph thuc hm...................................................................................97II.4- Cc tp ph thuc hm ti thiu.....................................................................................................98

III- Cc dng chun da trn kha chnh....................................................................99 III.1- Nhp mn v chun ho................................................................................................................99 III.2- Dng chun 1...............................................................................................................................101

III.3- Dng chun 2...............................................................................................................................102 III.4- Dng chun 3...............................................................................................................................103 III.5- Dng chun Boyce-Codd................. .............. ............... .............. .............. .............. ........... ..... ....104

IV- Cc thut ton thit k c s d liu quan h v cc dng chun cao hn......105IV.1- nh ngha tng qut cc dng chun.........................................................................................106IV.2- Cc thut ton thit k lc c s d liu quan h................................................................108

IV.2.1- Tch quan h v tnh khng y ca cc dng chun.....................................................................108IV.2.2- Php tch v s bo ton ph thuc....................................................................................................109IV.2.3- Php tch v kt ni khng mt mt...................................................................................................111

IV.3- Cc ph thuc hm a tr v dng chun 4.................................................................................117IV.3.1- nh ngha ph thuc a tr................................................................................................................118IV.3.2- Cc quy tc suy din i vi cc ph thuc hm v ph thuc a tr..................................................119IV.3.3- Dng chun 4 .....................................................................................................................................120IV.3.4- Tch c tnh cht ni khng mt mt thnh cc quan h 4NF............................................................121

IV.4- Cc ph thuc ni v dng chun 5.............................................................................................121

V- Tng kt chng v cu hi n tp........................................................................123V.1- Tng kt chng............................................................................................................................123

V.2- Cu hi n tp...............................................................................................................................124V.3- Bi tp...........................................................................................................................................126

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Chng I- CC KHI NIM C BN V H C SD LIU

Cc c s d liu v cc h c s d liu tr thnh mt thnh phn ch yutrong cuc sng hng ngy ca x hi hin i. Trong vng mt ngy con ngi cth c nhiu hot ng cn c s giao tip vi c s d liu nh: n ngn hng rt tin v gi tin, ng k ch trn my bay hoc khch sn, truy cp vo th vin tin hc ho tm sch bo, t mua tp ch mt nh xut bn Ti cc ngnhng, cc ca hng, ngi ta cng cp nht t ng vic qun l tin bc, hng ho.

Tt c cc giao tip nh trn c gi l cc ng dng ca c s d liutruyn thng. Trong cc c s d liu truyn thng, hu ht cc thng tin c lu

gi v truy cp l vn bn hoc s. Nhng nm gn y, nhng tin b v k thut a n nhng ng dng mi ca c s d liu. Cc c s d liu a phngtin by gi c th lu tr hnh nh, phim v ting ni. Cc h thng thng tin al c th lu tr v phn tch cc bn , cc d liu v thi tit v cc nh v tinh.Kho d liu v cc h thng phn tch trc tuyn c s dng trong nhiu cng ty ly ra v phn tch nhng thng tin c li t cc c s d liu rt ln nhm ara cc quyt nh. Cc k thut c s d liu ng v thi gian thc c s dngtrong vic kim tra cc tin trnh cng nghip v sn xut. Cc k thut tm kim c

s d liu ang c p dng cho World Wide Web cung cp vic tm kim ccthng tin cn thit cho ngi s dng bng cch duyt qua Internet.

hiu c cc c s k thut ca c s d liu chng ta phi bt u t ccc s k thut ca c s d liu truyn thng. Mc ch ca gio trnh ny lnghin cu cc c s k thut . Trong chng ny chng ta s nh ngha c sd liu, h qun tr c s d liu, m hnh c s d liu v cc thut ng c bnkhc.

I- C s d liu

I.1- nh ngha c s d liu

C s d liu v k thut c s d liu c nh hng rt ln n vic sdng my tnh. C th ni rng c s d liu ng vai tr quan trng trong mi

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lnh vc c s dng my tnh nh gio dc, thng mi, k ngh, khoa hc, thvin,. Thut ng c s d liu tr thnh mt thut ng ph dng.

Mt c s d liu l mt tp hp cc d liu c lin quan vi nhau, c lutr trn my tnh, c nhiu ngi s dng v c t chc theo mt m hnh. D

liu l nhng s kin c th ghi li c v c ngha.V d, qun l vic hc tp trong mt mi trng i hc, cc d liu l cc

thng tin v sinh vin, v cc mn hc, im thi.Chng ta t chc cc d liu thnh cc bng v lu gi chng vo s sch hoc s dng mt phn mm my tnh lu gi chng trn my tnh. Ta c mt tp cc d liu c lin quan n nhau vmang nhiu ngha, l mt c s d liu.

I.2- Cc tnh cht ca mt c s d liu

Mt c s d liu c cc tnh cht sau:1. Mt c s d liu biu th mt kha cnh no ca th gii thc nh hot

ng ca mt cng ty, mt nh trng, mt ngn hng Nhng thayi ca th gii thc phi c phn nh mt cch trung thc votrong c s d liu. Nhng thng tin c a vo trong c s d liuto thnh mt khng gian c s d liu hoc l mt th gii nh(miniworld) .

2. Mt c s d liu l mt tp hp d liu lin kt vi nhau mt cch logicv mang mt ngha c hu no . Mt c s d liu khng phi lmt tp hp tu tin.

3. Mt c s d liu c thit k v c ph bin cho mt mc ch ring.N c mt nhm ngi s dng c ch nh v c mt s ng dngc xc nh ph hp vi mi quan tm ca ngi s dng. Nicch khc, mt c s d liu c mt ngun cung cp d liu, mtmc tng tc vi cc s kin trong th gii thc v mt nhm

ngi quan tm tch cc n cc ni dung ca n.

Mt c s d liu c th c c tu v c phc tp thay i. C nhng cs d liu ch gm vi trm bn ghi (nh c s d liu phc v vic qun l lng mt c quan nh), v c nhng c s d liu c dung lng rt ln (nh cc cs d liu phc v cho vic tnh cc in thoi, qun l nhn s trn mt phm viln). Cc c s d liu phi c t chc qun l sao cho nhng ngi s dng c

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th tm kim d liu, cp nht d liu v ly d liu ra khi cn thit. Mt c s dliu c th c to ra v duy tr mt cch th cng v cng c th c tin hcho. Mt c s d liu tin hc ho c to ra v duy tr bng bng mt nhmchng trnh ng dng hoc bng mt hqun tr c s d liu.

II- H qun tr c s d liu

II.1- nh ngha h qun tr c s d liu

Mt h qun tr c s d liu l mt tp hp chng trnh gip cho ngi sdng to ra, duy tr v khai thc mt c s d liu. N l mt h thng phn mm

ph dng, lm d qu trnh nh ngha, xy dng v thao tc c s d liu cho ccng dng khc nhau.

nh ngha mt c s d liu bao gm vic c t cc kiu d liu, cc cutrc v cc rng buc cho cc d liu s c lu tr trong c s.

Xy dngmt c s d liu l qu trnh lu tr cc d liu trn cc phngtin lu tr c h qun tr c s d liu kim sot.

Thao tc mt c s d liu bao gm cc chc nng nh truy vn c s d liu ly ra cc d liu c th, cp nht c s d liu phn nh cc thay i trongth gii nh v to ra cc bo co t cc d liu.

Cc h qun tr c s d liu dng th hin mt c s d liu tin hc ho cth l ph dng (l mt phn mm ng gi) hoc c th l chuyn dng (l mttp cc phn mm c to ra vi mt mc ch ring).

Ngi ta gi c s d liu v h qun tr c s d liu bng mt thut ngchung l h c s d liu. Mi trng ca mt h c s d liu c m t bnghnh v di y (hnh I-1).

II.2- Cc chc nng ca mt h qun tr c s d liu

Mt h qun tr c s d liu hin nay c cc chc nng sau :1. Lu tr cc nh ngha, cc mi lin kt d liu (gi l siu d liu) vo

mt t in d liu. Cc chng trnh truy cp n c s d liu lmvic thng qua h qun tr c s d liu. H qun tr c s d liu sdng d liu trong t in d liu tm kim cc cu trc thnh

phn d liu v cc mi lin kt c yu cu. Mi s thay i trong

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cc tp c s d liu s c t ng ghi li vo t in d liu. Nhvy, h qun tr c s d liu gii phng ngi s dng khi vic lp

trnh cho cc mi lin kt phc tp trong mi chng trnh, vic sai cc chng trnh truy cp n tp c s d liu b sa i. Nicch khc, h qun tr c s d liu loi b s ph thuc gia d liuv cu trc ra khi h thng.

nh I-1. Mi trng h c s d liu

2. To ra cc cu trc phc tp theo yu cu lu tr d liu. N gipngi s dng lm nhim v kh khn l nh ngha v lp trnh chocc c trng vt l ca d liu.

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Ngi s dng / Ngi lp trnh

Chng trnh ng dng / Truy vn

Phn mm x lTruy vn / Chng trnh

Phn mm truy cp n ccd liu c lu tr

nh ngha c s d

liu (Siu d liu )C s d liu


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3. Bin i cc d liu c nhp vo ph hp vi cc cu trc d liu im 2. Nh vy, h qun tr c s d liu gip ngi s dng phn

bit dng logic v dng vt l ca d liu. Bng vic duy tr s clp d liu, h qun tr c s d liu chuyn cc yu cu logic thnhcc lnh nh v mt cch vt l v ly ra cc d liu yu cu. iu cng c ngha l h qun tr c s d liu to khun dng cho cc dliu c ly ra lm cho n ph hp vi mong mun logic cangi s dng.

4. To ra mt h thng bo mt v p t tnh bo mt v ring t trong c sd liu.

5. To ra cc cu trc phc tp cho php nhiu ngi s dng truy cp nd liu

6. Cung cp cc th tc sao lu v phc hi d liu m bo s an ton vton vn d liu.

7. Xc tin v p t cc quy tc an ton loi b vn ton vn d liu.iu cho php ta lm ti thiu s d tha d liu v lm ti atnh nht qun d liu.

8. Cung cp vic truy cp d liu thng qua mt ngn ng truy vn. Mtngn ng truy vn l mt ngn ng phi th tc cho php ngi s

dng ch ra ci g cn phi lm m khng cn phi ch ra n clm nh th no. Cc h qun tr c s d liu cng cung cp victruy cp d liu cho nhng ngi lp trnh thng qua cc ngn ngth tc.

II.3- Cc c trng ca gii php c s d liu

Trc khi khi nim c s d liu ra i, h thng tp (file) l mt phngphp c p dng trong vic qun l. Mt tp c th c xem l mt cp h s

lu tr cc thng tin lin quan n tng cng vic ring bit. V d, trong mt cquan, b phn ti v s c mt cp h s lin quan n lng ca cc nhn vin, b

phn t chc c cp h s lin quan n vn nhn s Vic x l ly ra ccthng tin nh l cc thng k v lng, v qu trnh cng tc lc u c thchin mt cch th cng. Dn dn, khi lng thng tin ngy cng ln, vic x lthng tin ngy cng phc tp, ngi ta s dng my tnh vo vic qun l. Cc cp

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h s c chuyn thnh cc tp trn my tnh v vic x l thng tin c thchin bng cch lp trnh (trong mt ngn ng lp trnh th h 3).

Vic qun l theo gii php h thng tp c rt nhiu nhc im. Th nht, l s d tha thng tin: cng mt thng tin c lu tr nhiu ln (chng hn,

danh sch nhn vin c mt trong tp lng v cng c mt c trong tp nhn s).iu gy ra vic lng ph b nh v d gy sai st trong khi cp nht d liu, dsinh ra cc d liu khng ng n. Th hai, l vic ph thuc gia chngtrnh ng dng v d liu. Mi khi c s thay i cu trc tp v cc d liu trongtp, chng trnh ng dng khai thc thng tin trn tp cng thay i theo. iu gy ra kh khn ln cho vic bo tr.

Gii php c s d liu ra i gii quyt c nhng nhc im . Cth, gii php c s d liu c nhng c trng sau:

1. Bn cht t m t ca h c s d liu.

Mt c trng c bn ca gii php c s d liu l h thng c s d liukhng ch gm c bn thn c s d liu m cn c c nh ngha hoc m t y v cu trc c s d liu v cc rng buc. nh ngha ny c lu tr trong tin h thng, n cha cc thng tin nh l cu trc ca mi tp, kiu v dng lutr ca tng mc d liu. Cc thng tin c lu gi trong t in gi l siu dliu (meta-data) v chng m t cu trc ca d liu nguyn thu (hnh I-1). Phn

mm h qun tr c s d liu v nhng ngi s dng c s d liu s dng tin ly thng tin v cu trc ca c s d liu.

2. S c lp gia chng trnh v d liu.

Trong h thng tp, cu trc ca cc tp c s d liu c nhng vo trongcc chng trnh truy cp, v vy bt k mt thay i no v cu trc ca mt tpcng i hi phi thay i tt c cc chng trnh truy cp n tp . Ngc li,cc chng trnh truy cp ca h qun tr c s d liu khng i hi vic thay inh th. Cu trc ca cc tp d liu c lu tr trong t in tch ri vi ccchng trnh truy cp. Tnh cht ny gi l s c lp d liu chng trnh.

3. H tr cc khung nhn d liu nhiu thnh phn.

Mt c s d liu c nhiu ngi s dng, mi mt ngi c th i hi mtphi cnh hoc mt khung nhn (view) khc nhau. Mt khung nhn c th l mttp con ca c s d liu hoc n c th cha cc d liu o, l cc d liu c

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trch ra t cc tp c s d liu khc nhau nhng khng c lu tr mt cch rrng. Mt h qun tr c s d liu nhiu ngi s dng phi cung cp nhiu cngc nh ngha cc khung nhn nhiu thnh phn.

4. Chia s d liu v nhiu ngi s dng.

Mt h qun tr c s d liu nhiu ngi s dng phi cho php nhiu ngis dng truy cp ng thi n c s d liu. H qun tr c s d liu phi c

phn mm kim tra cnh tranh m bo rng cc ngi s dng cp nht ncng mt c s d liu phi c thc hin theo cch c kim tra cho kt quca cc cp nht l ng n.

II.4- V d v mt c s d liu

Chng ta hy xem xt mt c s d liu m nhiu ngi quen bit: c s

d liu TRNG. C s d liu ny lu gi cc thng tin lin quan n sinh vin,cc mn hc, im trong mt mi trng i hc. C s d liu c t chcthnh 5 bng, mi bng lu tr cc bn ghi d liu cng mt kiu. BngSINHVIN lu gi d liu v cc sinh vin, bng MNHC lu gi cc d liuv cc mn hc, bng HCPHN lu gi cc d liu v cc hc phn ca cc mnhc, bng IM lu gi im ca tng hc phn ca cc sinh vin v bngBITTRC lu gi thng tin v cc mn hc cn bit trc ca cc mn hc.Cu trc ca c s d liu v mt vi mu d liu v d c trnh by hnh I-2.

SINHVIN MsSV HtnSV Lp Chuynngnh

17 Nguyn Nam K45T Tinhc

8 L Bc K45C CngnghTT

MNHC MsMH TnMH Svht Khoa

101 Tinhc c s 8 Cng ngh

102 Cu trc DL v GT 5 Cng ngh

103 Ton ri rc 5 Cng ngh104 C s d liu 3 Cng ngh

HCPHN MsHP MsMH Hck Nm Tngiovin

1011 101 1 2001 Vn

1012 101 2 2002 Vn

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1031 103 1 2001 Hong

1032 103 2 2002 Hong

1020 102 3 2002 Ln

1040 104 4 2002 Huy

IM MsSV MsHP im

17 1031 8

17 1020 6

8 1031 9

8 1011 10

8 1020 7

8 1040 9

BIT TRC M sMH MsMHbittrc

104 102

104 103

102 101

Hnh I-2. C s d liu TRNG

nh ngha c s d liu ny, chng ta phi ch ra cu trc ca cc bn ghi

ca mi tp (bng) bng cch c t cc kiu khc nhau ca cc phn t d liu sc lu tr trong mi bn ghi. Theo hnh I-2 , mi bn ghi SINHVIN bao gmcc d liu biu din M s sinh vin, H tn sinh vin, Lp, Chuyn ngnh.Mi bn ghi MNHC bao gm cc d liu biu din Tn mn hc, M smn hc, S n v hc trnh, Khoa, Chng ta phi ch ra mt kiu d liu chomi phn t d liu bn trong cc bn ghi. V d, ta c th c t H tn sinh vinl mt dy k t c di nh hn hoc bng 30, M s sinh vin l mt snguyn,.

xy dngc s d liu TRNG, chng ta lu gi cc d liu biudin mi sinh vin, mi mn hc, vo cc tp thch hp. rng cc bn ghitrong cc tp khc nhau c th c mi quan h vi nhau. V d, bn ghi i vi

Nguyn Nam trong tp SINHVIN c lin quan n hai bn ghi trong tp IM.Cc bn ghi ny ch ra im ca Nguyn Nam trong hai hc phn. Tng t nhvy, cc bn ghi trong tp c mi quan h vi cc bn ghi trong tp MNHC

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Thng thng mt c s d liu cha nhiu kiu bn ghi v cha nhiu mi quanh gia cc tp.

Thao tc c s d liu bao gm vic truy vn v cp nht c s d liu. Truyvn c s d liu l a ra cc yu cu i vi c s d liu ly ra cc thng tin

cn thit. V d, chng ta c th c cc truy vn nh: Lit k cc mn hc v imthi ca sinh vin Nguyn Nam, a ra danh sch cc sinh vin thi trt mn cs d liu. Cp nht c s d liu bao gm vic thm vo c s d liu bn ghi,xo b cc bn ghi hoc sa i cc gi tr trong cc bn ghi. Cc truy vn v cccp nht phi c c t trong ngn ng h c s d liu mt cch chnh xctrc khi chng c x l.

III- M hnh c s d liu

Cc loi cu trc c s d liu v mi lin h gia chng ng vai tr rt lntrong vic xc nh tnh hiu qu ca h qun tr c s d liu. V vy, thit k cs d liu tr thnh hot ng chnh trong mi trng c s d liu.

Vic thit k c s d liu c thc hin n gin hn nhiu khi ta s dngcc m hnh. Cc m hnh l s tru tng n gin ca cc s kin trong th giithc. Cc tru tng nh vy cho php ta kho st cc c im ca cc thc thv cc mi lin h c to ra gia cc thc th . Vic thit k cc m hnh tt sa ra cc c s d liu tt v trn c s s c cc ng dng tt. Ngc li, m

hnh khng tt s a n thit k c s d liu ti v dn n cc ng dng khngng.

Mt m hnh c s d liu l mt tp hp cc khi nim dng biu dincc cu trc ca c s d liu. Cu trc ca mt c s d liu l cc kiu d liu,cc mi lin kt v cc rng buc phi tun theo trn cc d liu. Nhiu m hnhcn c thm mt tp hp cc php ton c bn c t cc thao tc trn c s dliu.

III.1- Cc loi m hnh c s d liu

C rt nhiu m hnh d liu c ngh. Chng ta c th phn loi ccm hnh d liu da trn cc khi nim m chng s dng m t cc cu trc cs d liu.

Cc m hnh d liu bc cao hoc m hnh d liu mc quan nim cung cpcc khi nim gn lin vi cch cm nhn d liu ca nhiu ngi s dng. Cc m

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hnh ny tp trung vo bn cht logic ca biu din d liu, n quan tm n cic biu din trong c s d liu ch khng phi cchbiu din d liu.

Cc m hnh d liu bc thp hoc cc m hnh d liu vt l cung cp cckhi nim m t chi tit v vic cc d liu c lu tr trong my tnh nh th

no. Cc khi nim do m hnh d liu vt l cung cp ni chung c ngha ivi cc chuyn gia my tnh ch khng c ngha my i vi ngi s dngthng thng. gia hai loi m hnh ny l mt lp cc m hnh d liu th hin,chng cung cp nhng khi nim m ngi s dng c th hiu c v khng xavi cch t chc d liu bn trong my tnh. Ngi ta cn gi loi m hnh d liuny l loi m hnh d liu mc logic. Cc m hnh d liu th hin che giu mt schi tit v vic lu tr d liu nhng c th c ci t trc tip trn h thng mytnh.

Trong chng II, chng ta s nghin cu mt m hnh d liu mc quan nim,m hnh thc th - lin kt, gi tt l m hnh ER (Entity Relationship Model).M hnh ny s dng cc khi nim thc th, thuc tnh, mi lin kt, din tcc i tng ca th gii thc. Mt thc thdin t mt i tng hoc mt khinim ca th gii thc. V d, mt thc th l mt nhn vin hoc mt d n cm t trong c s d liu. Mt thuc tnh din t mt c trng no ca thcth. Chng hn, h tn, lng l cc thuc tnh ca thc th nhn vin. Mt milin kt gia hai hay nhiu thc th din t mt mi quan h qua li gia cc thc

th. V d, gia thc th nhn vin v thc th d n c mi lin kt mt nhn vinlm vic trn mt d n. M hnh d liu hng i tng cng l mt m hnh dliu bc cao. N s dng cc khi nim nh lp, phng thc, thng ip Bnc c th tm hiu v m hnh ny trong cc ti liu [1], [2].

Cc m hnh d liu th hin l cc m hnh c s dng thng xuyn nhttrong cc h c s d liu thng mi. Ba m hnh ni ting thuc loi ny l mhnh quan h, m hnh mng v m hnh phn cp. Cc m hnh mng v phn cp

ra i trc v c s dng rng ri trong qu kh (trc 1970). Vo u nhngnm 70, m hnh quan h ra i. Do tnh u vit ca n, m hnh quan h dn dnthay th cc m hnh mng v phn cp. Chng ta s nghin cu v m hnh quanh trong chng III.

Cc m hnh d liu vt l m t cch lu tr d liu trong my tnh gii thiucc thng tin nh khun dng bn ghi, sp xp bn ghi, ng truy cp

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III.2- Lc v trng thi c s d liu

Trong mt m hnh d liu cn phi phn bit r gia m t ca c s d liuv bn thn c s d liu. M t ca mt c s d liu c gi l lc c sd liu, n c xc nh r trong qu trnh thit k c s d liu v khng b thay

i thng xuyn. a s cc m hnh d liu c cc quy c hin th cc lc .Hin th ca mt lc c gi l biu ca lc . Mt biu lc ch th hin mt vi kha cnh ca lc nh l cc kiu bn ghi, cc mc d liuv mt s kiu rng buc. Cc kha cnh khc khng c th hin trong biu lc .

Cc d liu trong mt c s d liu c th thay i mt cch thng xuyn.Cc d liu trong mt c s d liu ti mt thi im c th c gi l mt trngthi c s d liu hoc l nh (snapshot) ca c s d liu. Nhiu trng thi quan

h c th c xy dng lm tng ng vi mt lc c s d liu c th.Mi khi chng ta chn vo hoc loi b mt bn ghi, sa i gi tr ca mt mc dliu trong mt bn ghi, chng ta lm thay i trng thi ca c s d liu sangmt trng thi khc.

Vic phn bit gia lc c s d liu v trng thi c s d liu l rtquan trng. Khi chng ta nh ngha mt c s d liu mi, ta ch c t lc cs d liu cho h qun tr c s d liu. Ti thi im ny, trng thi ca c s dliu l mt trng thi rng, khng c d liu. Chng ta nhn c trng thi banu ca c s d liu khi ta nhp d liu ln u tin. T tr i, mi khi mt

php ton cp nht c thc hin i vi c s d liu, chng ta nhn c mttrng thi c s d liu khc. Ti mi thi im, c s d liu c mt trng thihin ti. H qun tr c s d liu c trch nhim m bo rng mi trng thi c sd liu l mt trng thi vng chc, ngha l mt trng thi tho mn cu trc vcc rng buc c c t trong lc . V vy, vic c t mt lc ng ncho h qun tr c s d liu l mt vic lm cc k quan trng v lc phi

c thit k mt cch cn thn. H qun tr c s d liu lu tr cc m t ca cccu trc lc v cc rng buc cn gi l siu d liu vo trong t in(catalog) ca h qun tr sao cho phn mm h qun tr c s d liu c th thamkho n lc khi n cn. i khi ngi ta cn gi lc l mc tiu(intension) v trng thi c s d liu l m rng (extension) ca lc .

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IV- Con ngi trong h c s d liu

Vi mt c s d liu ln, rt nhiu ngi tham gia vo vic thit k, s dngv duy tr c s d liu. Nhng ngi lin quan n h c s d liu c chiathnh hai nhm chnh. Nhm th nht gm nhng ngi m cng vic ca h lin

quan hng ngy n c s d liu, l nhng ngi qun tr c s d liu, thit kc s d liu, s dng c s d liu, phn tch h thng v lp trnh ng dng.

Nhm th hai gm nhng ngi lm vic duy tr mi trng h c s d liunhng khng quan tm n bn thn c s d liu, l nhng ngi thit k vci t h qun tr c s d liu, pht trin cng c, thao tc vin v bo tr.

IV.1- Ngi qun tr h c s d liu (Database Administrator DBA)

Trong mt t chc c nhiu ngi cng s dng cc ti nguyn, cn phi c

mt ngi gim st v qun l. Trong mi trng h c s d liu, cc ti nguynl c s d liu, h qun tr c s d liu v cc phn mm lin quan. Ngi quntr h c s d liu l ngi chu trch nhim qun l cc ti nguyn . Ngi nychu trch nhim v vic cho php truy cp c s d liu, t chc v hng dnvic s dng c s d liu, cp cc phn mm v phn cng theo yu cu.

IV.2- Ngi thit k c s d liu (Database Designer)

Ngi ny chu trch nhim xc nh cc d liu s c lu gi trong c s,

chn cc cu trc thch hp biu din v lu gi cc d liu . Nhng nhimv ny c thc hin trc khi c s d liu c ci t v ph bin. Ngi thitk c trch nhim giao thip vi nhng ngi s dng tng lai hiu c cci hi ca h v a ra mt thit k tho mn cc yu cu . Anh ta cng cnhim v giao thip vi cc nhm ngi s dng v c kh nng h tr cc yu cuca cc nhm.

IV.3- Nhng ngi s dng (End User)

Nhng ngi s dng l nhng ngi m cng vic ca h i hi truy cpn c s d liu truy vn, cp nht v sinh ra cc thng tin. C th chia nhngngi s dng thnh hai nhm chnh: nhng ngi s dng th ng(tc l nhngngi s dng khng c nhiu kin thc v h c s d liu) v nhng ngi sdng ch ng(l nhng ngi c hiu bit tt v h c s d liu).

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Chc nng cng vic ca nhng ngi s dng th ng (chim phn lnnhng ngi s dng) gn lin vi vic truy vn v cp nht thng xuyn c sd liu bng cch s dng cc cu hi v cc cp nht chun (gi l cc giao tcnh sn) c lp trnh v kim tra cn thn. Nhng ngi ny ch cn hc mtt v cc phng tin do h qun tr c s d liu cung cp v hiu cc kiu giao tcchun c thit k v ci t l .

Nhng ngi s dng ch ng c hiu bit tt v h c s d liu, h c tht ci t cc ng dng ring ca mnh lm tho mn cc yu cu phc tp cah.

IV.4- Ngi phn tch h thng v lp trnh ng dng

Ngi phn tch h thng xc nh cc yu cu ca nhng ngi s dng (ch

yu l nhng ngi s dng th ng) c t cc chng trnh ph hp vi yucu ca h.

Ngi vit chng trnh ng dng th hin cc c t ca nhng ngi phntch thnh chng trnh, sau kim th, sa li lm ti liu v bo tr cc giao tcnh sn.

IV.5- Ngi thit k v ci t h qun tr d liu

l nhng ngi thit k, ci t cc m un, giao din ca h qun tr c s

d liu thnh cc phn mm ng gi. Mt h qun tr c s d liu l mt hthng phn mm phc tp bao gm nhiu thnh phn (m un). l cc m unci t t in d liu, ngn ng truy vn, b x l giao din, truy cp d liu,kim tra cnh tranh, phc hi v an ton. H qun tr c s d liu phi giao tipvi cc h thng phn mm khc nh h iu hnh v cc chng trnh dch chonhiu ngn ng khc nhau.

IV.6- Nhng ngi pht trin cng c

L nhng ngi thit k v ci t cc cng c (tool), l cc phn mmng gi lm d vic thit k v s dng c s d liu.

IV.7- Cc thao tc vin v nhng ngi bo tr

L nhng ngi chu trch nhim v vic chy v bo tr phn cng v phnmm ca h thng.

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V- Ngn ng c s d liu v giao din

V.1- Cc ngn ng h qun tr c s d liu

Mt khi vic thit k c s d liu hon thnh, cn phi chn mt h quntr c s d liu ci t c s d liu. Trong cc h qun tr c s d liu hinnay thng c cc ngn ng: ngn ng nh ngha d liu (data definition language

DDL) v ngn ng thao tc d liu (data manipulation language DML).

Ngn ng nh ngha d liu c s dng nh ngha cc lc . Hqun tr c s d liu c mt chng trnh dch ngn ng DDL, nhim v ca n lx l cc cu lnh DDL xc nh m t ca cu trc lc v lu tr m tlc vo t in ca h qun tr c s d liu.

Ngn ng thao tc c s d liu c s dng thao tc c s d liu. Cc

thao tc chnh gm c ly ra, chn vo, loi b v sa i cc d liu. C hai kiungn ng thao tc d liu chnh: ngn ng thao tc d liu mc cao hoc ngn ng

phi th tc hoc ngn ng thao tc d liu mc thp.

Ngn ng thao tc d liu mc cao c th c s dng din t cc phpton c s d liu mt cch ngn gn. Phn ln cc h qun tr c s d liu cho

php nhp cc lnh ca ngn ng thao tc d liu mc cao theo cch lp (ngha lsau khi nhp mt lnh, h thng s thc hin lnh ri mi nhp lnh tip theo)

hoc c nhng vo mt ngn ng lp trnh vn nng. Trong trng hp nhngvo ngn ng khc, cc lnh ca ngn ng thao tc d liu phi c xc nh bntrong chng trnh sao cho mt chng trnh tin dch c th nhn ra chng vc h qun tr c s d liu x l.

Ngn ng thao tc c s d liu mc thp hoc ngn ng th tc phi cnhng vo trong mt ngn ng lp trnh vn nng. Ngn ng thao tc c s d liukiu ny thng rt ra cc bn ghi hoc cc i tng ring r v x l chng mtcch ring r. V vy, chng cn phi s dng cc cu trc ngn ng lp trnh nh

vng lp, iu kin, rt ra tng bn ghi mt t mt tp cc bn ghi. Ngn ngthao tc d liu mc thp c gi l ngn ng mt ln mt bn ghi. Cc ngnng thao tc d liu mc cao c th dng mt lnh rt ra mt lc nhiu bn ghinn chng c gi l ngn ng mt ln mt tp hp.

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V.2- Cc loi giao din h qun tr c s d liu

Cc h qun tr c s d liu cung cp rt nhiu loi giao din ngi dngthn thin. Cc loi giao din chnh gm c:

Giao din da trn bng chn:Cc giao din ny cung cp cho ngi s dng

danh sch cc la chn, gi l bng chn (menu) v hng dn ngi s dng dint mt yu cu t u n cui. Cc bng chn lm cho ngi s dng khng cnnh cc lnh v c php ca ngn ng truy vn. Cc bng chn th xung trthnh k thut ph bin trong cc giao din da trn ca s. Chng thng c sdng trong cc giao din qut, cho php ngi s dng nhn thy ni dung ca mtc s d liu theo cch khng c cu trc.

Giao din da trn mu biu: Cc giao din ny hin th mt mu biu chongi s dng. Nhng ngi s dng c th in vo tt c cc ca mu biu nhp cc d liu mi hoc h ch in vo mt s cn h qun tr c s d liu sa ra cc d liu ph hp cho cc khc. Cc mu biu thng c thit k vc lp trnh cho cc ngi dng n gin. Mt s h thng c cc tin ch gipngi s dng tng bc xy dng mt mu biu trn mn hnh.

Giao din ho: Mt giao din ho (GUI) thng hin th mt lc cho ngi s dng di dng biu . Ngi dng c th thc hin mt truy vn

bng cch thao tc trn biu . Trong nhiu trng hp, GUI s dng c cc bng

chn v cc mu biu. a s cc GUI s dng cc cng c tr nh chut, phm kch cc phn ca s .

Giao din cho ngi qun tr h thng: a s cc h qun tr c s d liu ccc lnh u tin, ch c nhng ngi qun tr h thng mi s dng cc lnh . l cc lnh to ra cc ti khon (account), t cc tham s cho h thng, cp cc tikhon, thay i lc hoc t chc li cc cu trc lu tr ca c s d liu.

VI- Cu hi n tp

1. nh ngha cc thut ng : c s d liu, h qun tr c s d liu, h c sd liu, t in c s d liu, m hnh c s d liu.

2. Nu cc tnh cht ca mt c s d liu

3. Nu cc chc nng ca mt h qun tr c s d liu

4. Gii thch cc c trng ca gii php c s d liu

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5. nh ngha m hnh c s d liu v phn loi

6. Lit k cc ngi c lin quan n h c s d liu.

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Chng II- M HNH THC TH - LIN KT

Trong chng ny chng ta s lm quen vi m hnh thc th - lin kt, gi ttl m hnh ER ( Entity-Relationship Model). l mt m hnh d liu mc quan

nim ph bin, tp trung vo cc cu trc d liu v cc rng buc. M hnh nythng c s dng thit k cc ng dng c s d liu v nhiu cng c thitk c s d liu s dng cc khi nim ca n.

I- S dng m hnh quan nim bc cao cho vic thit k c s dliu

Qu trnh thit k mt c s d liu s dng m hnh quan nim bc cao c

minh ha bng hnh II.1.Bc u tin l tp hp cc yu cu v phn tch. Trong bc ny, ngi

thit k c s d liu phng vn nhng ngi s dng c s d liu hiu v lmti liu v cc yu cu v d liu ca h. Kt qu ca bc ny l mt tp hp ghichp sc tch v cc yu cu ca nhng ngi s dng. Nhng yu cu s c ct cng y v chi tit cng tt. Song song vi vic c t cc yu cu d liu,cn phi c t cc yu cu v chc nng ca ng dng: l cc thao tc do ngis dng nh ngha s c p dng i vi c s d liu.

Mi khi tt c cc yu cu c thu thp v phn tch, bc tip theo l tora lc quan nim cho c s d liu bng cch s dng m hnh d liu quannim mc cao. Bc ny gi l thit k quan nim. Lc quan nim l mt mt sc tch v cc yu cu d liu ca nhng ngi s dng. N bao gm cc m tchi tit ca cc kiu thc th, kiu lin kt v cc rng buc, chng c biu din

bng cc khi nim do cc m hnh d liu bc cao cung cp. V nhng khi nimny khng cha cc chi tit ci t, chng thng d hiu v c th s dng chng giao lu vi nhng ngi s dng. Lc quan nim mc cao cng c thc s dng nh mt dn chng m bo rng tt c cc i hi ca ngi sdng u tha mn v cc i hi ny khng cha cc mu thun. Gii php nycho php nhng ngi thit k c s d liu tp trung vo vic c t cc tnh chtca d liu m khng cn quan tm n cc chi tit lu tr. Mt thit k c s dliu quan nim tt s lm d cho cng vic ca nhng ngi thit k c s d liu.

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Trong qu trnh (hoc sau khi) thit k lc quan nim, chng ta c th sdng cc php ton c bn ca m hnh d liu c t cc thao tc ca ngi sdng c xc nh trong khi phn tch chc nng. iu cng gip khng nhrng lc quan nim tha mn mi yu cu chc nng c xc nh. Nu cmt s yu cu chc nng khng th nu ra c trong lc ban u th bcny c th c s sa i lc quan nim cho ph hp.

Bc tip theo trong vic thit k c s d liu l vic ci t mt c s dliu bng cch s dng mt h qun tr c s d liu c sn. Hu ht cc h qun trc s d liu s dng mt m hnh d liu ci t (th hin), chng hn nh mhnh quan h hoc i tng, v vy lc quan nim c chuyn t m hnh dliu bc cao thnh m hnh d liu ci t. Bc ny gi l thit k logic hoc lnh x m hnh d liu. Kt qu ca bc ny l mt lc c s d liu di

dng mt m hnh d liu ci t ca h qun tr c s d liu.Bc cui cng trong thit k c s d liu l thit k vt l. Trong bc ny

ta phi ch ra cc cu trc bn trong, cc ng dn truy cp, t chc tp cho cctp c s d liu. Song song vi cc hot ng , cc chng trnh ng dng cngc thit k v ci t nh l cc giao tc (transaction) c s d liu tng ngvi cc c t giao tc mc cao.

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Hnh II-3. S m t cc bc chnh ca vic thit k

II- Cc thnh phn c bn ca m hnh ER

II.1- Thc th v thuc tnh

i tng c trnh by trong m hnh ER l thc th. Thc thl mt vttrong th gii thc, c s tn ti c lp. Mt thc th c th l c th, tc l chngta c th cm nhn c bng cc gic quan, hoc c th l tru tng, tc l cim chng ta khng cm nhn c bng cc gic quan nhng c th nhn bit c

Th gii thc

TP HP V PHN TCHCC YU CU

THIT K QUAN NIM

THIT K L GIC

THIT K VT L

PHN TCH CHC NNG

THIT K CHNG TRNHNG DNG

CI T GIAO TC

Cc yu cu chc nng

c t giao tc mc cao

Cc yu cu c s d liu

Lc quan nim

Lc l gic

(c lp vi h QTCSDL)

Cc chng trnh ng dng

Lc bn trong

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bng nhn thc. Mt ci t, mt nhn vin, l nhng thc th c th. Mt nv cng tc, mt trng hc l nhng thc th tru tng.

Mi mt thc th c cc thuc tnh, l cc c trng c th m t thc th. V d, mt thc th Nhnvin c m t bngHtn, Tui, ach, Lng

ca nhn vin . Mt thc th c th s c mt gi tr cho mi thuc tnh ca n.V d, nhn vin nv1 c cc gi tr cho cc thuc tnh Htn, Tui, ach, Lngca n l L Vn, 32, H ni, 500000. Cc gi tr thuc tnh m t mi thcth s tr thnh mt phn chnh ca cc d liu s c lu gi trong c s d liu.Trong m hnh ER c mt nhiu kiu thuc tnh: thuc tnh n, thuc tnh phchp, thuc tnh n tr, thuc tnh a tr, thuc tnh c lu tr, thuc tnh suydin c, thuc tnh c gi tr khng xc nh, thuc tnh phc tp.

Thuc tnh n l thuc tnh khng th phn chia ra c thnh cc thnh

phn nh hn. V d, thuc tnh Tui ca mt nhn vin l mt thuc tnh n.Thuc tnh phc hp l thuc tnh c th phn chia c thnh cc thnh phn nhhn, biu din cc thuc tnh c bn hn vi cc ngha c lp. V d, thuc tnhHtn ca thc th nhn vin c th phn chia thnh cc tnh Hm v Tn. Gitr ca mt thuc tnh l s kt hp kt hp cc gi tr ca cc thuc tnh thnh

phn to nn n. Vic phn chia mt thuc tnh phc hp thnh cc thuc tnh nty thuc vo hon cnh c th.

Nhng thuc tnh c gi tr duy nht cho mt thc th c th gi l cc thuctnh n tr. V d, Htn l mt thuc tnh n tr ca thc th nhn vin, minhn vin c mt h tn duy nht. Trong mt s trng hp, mt thuc tnh c thc mt tp gi tr cho cng mt thc th. Nhng thuc tnh nh vy gi l thuctnh a tr. V d, thuc tnh Bngcp ca mt ngi. Mt ngi c th khng c

bng cp no, ngi khc c th c mt bng, ngi khc na c th c nhiubng. Nh vy, cc ngi khc nhau c th c mt s gi tr khc nhau cho thuctnh Bngcp. Thuc tnh Bngcp l mt thuc tnh a tr.

Thuc tnh c lu trl cc thuc tnh m gi tr ca n c nhp vo khici t c s d liu. Trong mt s trng hp, hai hay nhiu thuc tnh c gi trlin quan n nhau. V d, thuc tnh Tui v thuc tnh Ngysinh ca mt ngi.Vi mt ngi c th, ta c th tnh tui ca anh ta bng cch ly nm hin ti tri nm ca Ngysinh. Thuc tnh m gi tr ca n c th tnh c thng qua gitr ca cc thuc tnh khc gi l thuc tnh suy din c.

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Cc gi tr khng xc nh (null values): Trong mt s trng hp, mt thcth c th c th khng c cc gi tr p dng c cho mt thuc tnh. V d,thuc tnh Sinthoi ca thc th nhn vin s khng c gi tr i vi cc nhnvin khng c s in thoi. Trong trng hp nh vy, ta phi to ra mt gi trc bit gi l gi tr khng xc nh (null). Gi tr khng xc nh c to ra khimt thuc tnh c gi tr khng p dng c hoc khi khng bit.

Cc thuc tnh phc tp: L s kt hp ca cc thuc tnh phc hp v a tr.

II.2- Kiu thc th, tp thc th, kha v tp gi tr

Cc kiu thc th v cc tp thc th: Mt c s d liu thng cha nhngnhm thc th nh nhau. V d, mt cng ty thu hng trm nhn vin v lu ginhng thng tin tng t lin quan n mi nhn vin. Cc thc th nhn vin ny

chia s cc thuc tnh ging nhau nhng mi thc th c cc gi tr ring cho ccthuc tnh . Mt kiu thc thl mt tp hp cc thc th c cc thuc tnh nhnhau. Mt kiu thc th trong c s d liu c m t bng tn v cc thuc tnh.Vd: NHNVIN (Htn, Tui, Lng), CNGTY (Tn, aim, Gimc).Mt tp hp cc thc th ca mt kiu thc th c th trong c s d liu ti mtthi im c gi l mt tp thc th, n thng c tham chiu n bng cchs dng tn ca kiu thc th. V d, NHNVIN va dng ch mt kiu thcth, va ch tp hp hin ti ca tt c cc thc th nhn vin trong c s d

liu. Hnh II-2 minh ha cc kiu thc th NHNVIN, CNGTY v cc tp thcth tng ng.

Mt kiu thc th c biu din trong lc ER nh l mt hp hnh chnht c cha tn kiu thc th. Cc thuc tnh c t trong cc hnh van vc ni vi cc kiu thc th bng cc ng thng. Cc thuc tnh phc hpcng c ni vi cc thuc tnh thnh phn ca n bng ng thng. Cc thuctnh a tr c hin th trong cc hnh van p (hnh II-3).

Mt kiu thc th m t mt lc (hoc mt mc ch) cho mt tp ccthc th chia s cng mt cu trc. Tp hp cc thc th ca mt kiu thc th cth c nhm vo mt tp thc th v c gi l mt th hin ca mt kiu thcth.

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NHNVIN(Htn, Tui, Lng)

CNGTY(Tn, adim, Gimc)

Nv1

(L Lan, 30, 800000)

Nv2(Trn B, 45, 1000000)

Nv3

(Hong Vn, 25, 600000)

.

.

.

Ct1

(CT Phn mm, H ni, V An)

Ct2(CT Hoa qu, Hi phng, L H)

Ct3

(CT My tnh, H ni, PhanAnh)

.

.

.

Hnh II-4. Kiu thc th v tp thc th

Thuc tnh kha ca mt kiu thc th: Mt rng buc quan trng trn ccthc th ca mt kiu thc th l kha. Mt kiu thc th thng c mt thuc tnhm cc gi tr ca n l khc nhau i vi mi thc th ting bit trong mt tpthc th. Thuc tnh nh vy gi l thuc tnh kha v cc gi tr ca n c th

dung xc nh tng thc th mt cch duy nht. V d, thuc tnh Tn ca kiuthc th CNGTY l kha ca kiu thc th v mi thc th cng ty c mt tnduy nht. i khi, nhiu thuc tnh kt hp vi nhau to thnh mt kha, ngha lt hp cc gi tr ca cc thuc tnh ny phi khc nhau i vi mi thc th. Trongtrng hp nh vy ta c mt thuc tnh kha phc hp. Ch rng kha phc hp

phi ti thiu, ngha l tt c cc thuc tnh thnh phn phi c mt trong thuc tnhphc hp tha mn tnh cht duy nht. Trong biu ha ca m hnh ER,thuc tnh kha c biu din bng cch gch ngang di tn ca n (hnh II-3).

Khi ch ra rng mt thuc tnh l kha ca mt kiu thc th ngha l tnh chtduy nht nu trn phi c tha mn i vi i vi mi m rng ca kiu thcth. Nh vy, rng buc kha cm hai thc th bt k c gi tr cho thuc tnh khanh nhau ti cng mt thi im. l mt rng buc trn tt c cc th hin cathc th. Rng buc kha cng nh cc rng buc s c gii thiu v sau cly ra t cc rng buc ca th gii nh ca c s d liu.

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Mt kiu thc th c th c nhiu hn mt thuc tnh kha. V d, nu mtcng ty c mt m s duy nht v mt tn duy nht th cc thuc tnh Mscng tyv Tncngty u l cc thuc tnh kha. Mt kiu thc th cng c th khng ckha. Mt thc th khng c kha c gi l kiu thc th yu.

Hnh II-5. Biu din kiu thc th v cc thuc tnh

Min gi tr ca cc thuc tnh: Mi thuc tnh n ca mt kiu thc thc kt hp vi mt min gi tr. l mt tp cc gi tr c th gn cho thuctnh ny i vi mi thc th ring bit. Cc min gi tr khng hin th trong ccs ER.

Mt cch ton hc, mt thuc tnh A ca kiu thc th E c tp gi tr V cth c nh ngha nh l mt hm t E vo tp hp lc lng P(V) ca V:

A: E P(V)

Ta k hiu gi tr ca thuc tnh A i vi thc th e l A(e). nh ngha trn ng cho cc thuc tnh n tr, a tr v thuc tnh khng xc nh. Mt gi trkhng xc nh c biu din bng mt tp rng. Vi cc thuc tnh n tr, A(e)l mt gi tr n cho thc th e. Cc thuc tnh a tr khng c cc hn ch trn

A(e). Vi mt thuc tnh phc hp A, tp gi tr V l tch cc ca P(V1)xP(V2)x.xP(Vn), trong V1, V2, , Vn l tp cc gi tr cho cc thnh phn nca A.

II.3- Kiu lin kt, tp lin kt v cc th hin

Mt kiu lin ktR gia n kiu thc th E1, E2, ,En xc nh mt tp lin ktgia cc thc th ca cc kiu . Cng nh cc kiu thc th v tp thc th, mt

NHNVIN

MsNV Htn Ngysinh Bngcp

Hm Tn

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kiu lin kt v tp lin kt tng ng vi n cng c tn chung l R. Mt cchton hc, tp lin kt R l mt tp hp cc th hin lin kt ri , i= 1,2, trong mi ri lin kt n thc th ring bit e1,e2,,en v mi mt thc th ej trong ri l mtthnh phn ca kiu thc th Ej , 1 j n. Nh vy, mt kiu lin kt R l mt quanh ton hc trn E1, E2, , En hoc c th nh ngha nh l mt tp con ca tch cc E1 x E2 x x En. Mi kiu thc th E1,E2, , En c gi l tham gia vokiu lin kt R, v tng t, mi thc th ring bit e1, e2, , en c gi l thamgia vo th hin lin kt ri = ( e1,e2,,en).

Mt cch khng hnh thc, mi th hin lin kt ri trong R l mt s kt hpca cc thc th, mi thc th thuc v mt kiu thc th tham gia vo lin kt.Mi lin kt ri nh vy din t mt s kin rng cc thc th tham gia trong ri cmi quan h vi nhau theo mt cch no trong th gii thc. V d, trong thc

t, cc nhn vin lm vic cho cc n v, nh vy, c mt kiu lin kt lin ktlm vic cho, lin kt gia kiu thc th NHNVIN v kiu thc th NV.

Trong s ER, kiu lin kt c biu din bng mt hnh thoi ni trc tipvi cc hnh ch nht biu din cc kiu thc th tham gia vo lin kt. Hnh II-4minh ha kiu lin kt v th hin lin kt

a) Kiu lin kt

b) Th hin lin kt

Hnh II-6. Kiu lin kt v th hin lin kt

NHNVIN NV

Lm vic

cho

Nv1Nv2

Nv3Nv4

.

.

v1

v2

NHNVINlm vic cho

NV

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II.4- Cp lin kt, tn vai tr v kiu lin kt quy

Cp ca mt kiu lin ktl s cc kiu thc th tham gia vo kiu lin kt .Mt kiu lin kt c th c cp 1, cp 2, cp 3,. V d, kiu lin kt gia kiu thc th NHNVIN v kiu thc th NV l mt kiu lin kt

cp 2. Kiu lin kt gia kiu thc th MNHC vi chnh n l mtkiu lin kt cp 1

i khi chng ta c th coi mt kiu lin kt nh mt thuc tnh ca mt kiuthc th. V d, nu kiu thc th NHNVIN c thuc tnh nv ch ra tnn v m nhn vin lm vic cho, th thuc tnh nv biu th mt kiu lin kt.

Ni cch khc, mt thuc tnh ca mt kiu thc th hoc c chc nng biu thmt c trng ca kiu thc th, hoc c chc nng biu th mt kiu lin kt giakiu thc th vi cc kiu thc th khc. Cc thuc tnh biu th mt kiu lin

kt c th n tr hoc a tr tu theo bn cht ca mi lin kt.

Cc tn vai tr v cc kiu lin kt quy: Mi mtkiu thc th tham giavo mt kiu lin kt c mt vai tr c th trong lin kt. Tn vai tr dng ch rvai tr ca cc thc th ca kiu thc th tham gia lin kt, n gip vic giithch ngha ca lin kt. V d, trong kiu lin kt NHNVIN NV, vai tr ca cc thc th ca kiu thc th NHNVIN l nhn vin hoccng nhn cn vai tr ca cc thc th ca kiu thc th NV l n v hoc nithu cng nhn. Nu cc kiu thc th tham gia vo kiu lin kt l khc nhau thtn vai tr l hon ton khng cn thit bi v c th s dng tn cc kiu thc thlm tn vai tr. Tuy nhin, trong mt s trng hp, mt kiu thc th c th thamgia vo mt kiu lin kt vi cc vai tr khc nhau. Trong nhng trng hp nhvy, tn vai tr tr nn cn thit phn bit ngha ca mi s tham gia. Cc kiulin kt nh vy gi l kiu lin kt quy. V d, trong s cc nhn vin lm viccho mt n v, c cc nhn vin c phn cng gim st cc nhn vin khc.

Nh vy s c mt kiu lin kt gia cc thc th ca kiu thc th NHNVIN:

NHNVIN NHNVIN. Kiu thc th NHNVIN tham gia hailn vo kiu lin kt , mt ln vi vai tr ngi gim st, mt ln vi vaitr ngi b gim st.

II.5- Cc rng buc trn cc kiu lin kt

Cc kiu lin kt thng c mt s rng buc hn ch s cc t hp c thca cc thc th c th tham gia trong tp hp lin kt tng ng. Cc rng buc

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ny c xc nh t tnh trng ca th gii thc m kiu lin kt biu din. V d,nu cng ty c quy ch l mt nhn vin ch lm vic cho mt n v th chng ta

phi m t rng buc ny trong lc . C hai loi rng buc chnh: t s lclng v s tham gia.

T s lc lng: T s lc lng cho mt kiu lin kt ch ra s cc th hinlin kt m mt thc th c th tham gia. Vi cc kiu lin kt cp 2, c th c cct s lc lng 1:1, 1:N, v M:N. Mt kiu lin kt c t s lc lng 1:1 gia haikiu thc th A v B c ngha l trong kiu lin kt , mt thc th ca kiu A chlin kt vi mt thc th ca kiu B v ngc li, mt thc th ca kiu B ch linkt vi mt thc th ca kiu A. T s lc lng 1:N c ngha l mt thc th cakiu A c th lin kt vi nhiu thc th ca kiu B nhng mt thc th ca kiu Bch lin kt vi mt thc th ca kiu A. Trong kiu lin kt c t s lc lng

M:N, mi thc th ca kiu A c th lin kt vi nhiu thc th ca kiu B vngc li, mi thc th ca kiu B c th lin kt vi nhiu thc th ca kiu A.Trong biu din ca lc ER, cc t s lc lng c biu din bng cch ghi1, N, M trn cc hnh thoi biu din kiu lin kt (hnh II-5) .

Hnh II-7. T s lc lng ca cc kiu lin kt

Cc rng buc tham gia v s ph thuc tn ti: Rng buc tham gia ch rarng c phi s tn ti ca mt kiu thc th ph thuc vo mt kiu thc th khc

thng qua mt kiu lin kt hay khng. C hai kiu rng buc tham gia: rng buctham gia ton b v rng buc tham gia b phn. Tham gia ton b ngha l tt ccc thc th ca kiu thc th phi tham gia vo kiu lin kt cn tham gia b phnngha l ch mt b phn cc thc th ca kiu thc th tham gia vo kiu lin kt.V d, xt kiu lin kt NHNVIN NV. Trong thc t, mi n v

phi c mt ngi qun l (l mt nhn vin) nhng khng phi nhn vin nocng qun l mt n v. Nh vy, s tham gia ca cc thc th n v vo kiu

NHNVIN NVQun l

NHNVIN NVLm vic cho

NHNVIN DNLm vic trn

1 1

N 1

M N

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lin kt l ton b cn s tham gia ca cc thc th nhn vin vo kiu lin kt lb phn. S tham gia ton b cn c gi ls ph thuc tn ti.

Trong lc ER, s tham gia ton b c biu th bng ng ni i tkiu thc th n kiu lin kt. V d :

II.6- Thuc tnh ca cc kiu lin kt

Cc kiu lin kt cng c th c cc thuc tnh, ging nh cc thuc tnh cacc kiu thc th. V d, kiu lin kt gia cc kiu thc th

NHNVIN v DN c th c thuc tnh Sgi ghi li s gi lm vic camt nhn vin trn mt d n. Cc thuc tnh ca kiu lin kt cng c biu din

bng mt hnh van v c ni vi kiu lin kt. V d:

II.7- Cc kiu thc th yu

Cc kiu thc th khng c cc thuc tnh kho cho chnh mnh c gi lcc kiu thc th yu. Ngc li, cc kiu thc th thng thng (ngha l c thuctnh kho) c gi l kiu thc th mnh. Cc thc th ca mt kiu thc th yuc xc nh bng cch lin kt vi cc thc th c th ca mt kiu thc th khc

phi hp vi mt s gi tr thuc tnh ca n. Ta gi kiu thc th khc l kiuthc th xc nh hoc kiu thc th ch. Ta gi kiu lin kt gia kiu thc th yuv kiu thc th ch ca n l lin kt xc nh ca thc th yu. Mt kiu thc thyu lun lun c mt rng buc tham gia ton b (tn ti ph thuc) vo lin kt

xc nh ca n bi v mt kiu thc th yu khng th c xc nh m khng ckiu thc th ch. V d, trong mt cng ty, con ca nhn vin v nhn vin c thhng ch bo him theo nhn vin. Nh vy, s c mt kiu lin kt

NHNVIN < CON>. y l mt kiu lin kt c t s lc lng 1:N. Ccthuc tnh ca kiu thc th CON l Htn, Ngaysinh, Giitnh. Hai ngi con cahai nhn vin khc nhau c th c cng gi tr cho cc thuc tnh nhng n l haithc th khc nhau. Chng ch c xc nh nh hai thc th khc nhau sau khi

NHNVIN DNLm vic trn1 1

S

gi

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xc nh mt thc th nhn vin c th c lin quan n tng ngi ph thuc. Mithc th ca kiu thc th NHNVIN c gi l ch ca cc thc th ca kiuthc th CON lin kt vi n.

Thng thng, cc kiu thc th yu c mt kho b phn, l mt tp hp

cc thuc tnh c th xc nh mt cch duy nht cc thc th yu lin kt vi cngmt thc th ch. V d, nu hai ngi con ca mt nhn vin khng bao gi c tnging nhau th thuc tnh Htn ca kiu thc th CON l mt kho b phn. Trongtrng hp xu nht, thuc tnh phc hp gm tt c cc thuc tnh ca thc thyu s l mt kho b phn.

Trong s ER, kiu thc th yu v kiu lin kt xc nh ca n c biudin bng mt hnh ch nht v mt hnh thoi nt i. V d:

III- V d v thit k m hnh ER

Trong phn ny, chng ta xt v d v vic xy dng m hnh ER cho c s dliu cng ty. Nh trong phn I ni, bc u tin trong vic thit k mt c sd liu l tp hp v phn tch cc yu cu. Kt qu ca bc ny l mt tp hpcc ghi chp sc tch v cc yu cu ngi s dng cng nh tnh trng ca ni ta

cn xy dng c s d liu.Gi s rng sau khi tp hp cc yu cu v phn tch, hot ng ca cng ty

c ghi chp li nh sau:

1. Cng ty c t chc thnh cc n v. Mi n v c mt tn duy nht,mt m s duy nht, mt nhn vin c th qun l n v. Vic nhn vin qun ln v c ghi li bng ngy nhn vin bt u qun l. Mt n v c th cnhiu a im.

2. Mi n v kim sot mt s d n. Mt d n c mt tn duy nht, mt ms duy nht v mt a im.

3. Vi mi nhn vin chng ta lu gi li H tn, M s, a ch, lng, giitnh, ngy sinh. Mt nhn vin ch lm vic cho mt n v nhng c th lm victrn nhiu d n do nhiu n v kim sot. Chng ta lu gi li s gi lm vic

NHNVIN CONC1 N

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ca mi nhn vin trn mt d n. Mi nhn vin c th c mt ngi gim st trctip, ngi cng l mt nhn vin.

4. Mi nhn vin c nhng ngi con. Nhng ngi ny c hng bohim theo nhn vin. Vi mi ngi con ca nhn vin, chng ta lu gi H tn,

gii tnh, ngy sinh .III.1- Xc nh cc kiu thc th, cc thuc tnh v cc kiu lin kt

Theo cc ghi chp trn, chng ta c th xc nh cc kiu thc th v cckiu lin kt nh sau:

1. CNGTY khng phi l mt kiu thc th v y ta c mt cng ty duynht.

2. NV l mt kiu thc th vi cc thuc tnh Tn, Ms, Ngiqunl,Ngybtu v aim. Cc thuc tnh Tn, Ms, aim l cc thuc tnh mt n v, cc thuc tnh Ngiqunl, Ngybtu l cc thuc tnh biu th mtkiu lin kt (vi kiu thc th NHNVIN). Cc thuc tnh u l n v n tr,tr thuc tnh aim, n l mt thuc tnh a tr (mt n v c nhiu a im).Cc thuc tnh Tn, Ms l cc thuc tnh kha (v mi n v c mt tn v mtm s duy nht).

3. Kiu thc th DN c cc thuc tnh Tn, Ms, aim,

nvkimsot. Cc thuc tnh Tn, Ms, a im l cc thuc tnh m tDN, thuc tnh nvkimsot biu th kiu lin kt vi kiu thc th NV(mt n v kim sot mt s d n). Cc thuc tnh Tn, Ms l cc thuc tnhkha.

4. Kiu thc NHNVIN vi cc thuc tnh Htn, Ms, Gii tnh,Ngysinh, Lng, nv, Ngigimst. Thuc tnh Htn l mt thuc tnh phchp (gm Hm, Tn). Cc thuc tnh nv, Ngigimst m t cc kiu linkt gia kiu thc th NHNVIN v cc kiu thc th NV v NHNVIN

tng ng. Thuc tnh Ms l thuc tnh kha.

5. Kiu thc th CON vi cc thuc tnh Nhnvin, Htn, Gii tnh,Ngysinh. Thuc tnh Nhnvin m t kiu lin kt vi kiu thc th NHNVIN.

6. Kiu lin kt NV DN l kiu lin kt c t s lclng 1:N (mt n v kim sot mt s d n, mt d n do mt n v qun l).S tham gia ca DN vo kiu lin kt l ton b (bi v d n no cng c

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mt n v kim sot). Nu n v no cng c d n th vic tham gia ca NVvo kiu lin kt l ton b, ngc li s tham gia l b phn.

7. Kiu lin kt NHNVIN NV c t s lc lng N:1(mi nhn vin lm vic cho mt n v nhng mi n v c nhiu nhn vin l

vic). S tham gia ca hai kiu thc th vo lin kt l ton b.8. Kiu lin kt NHNVIN NV c t s lc lng 1:1 (mt

nhn vin qun l mt n v v mt n v c mt nhn vin qun l). S thamgia ca kiu th NHNVIN vo kiu lin kt l b phn (bi v khng phi nhnvin no cng qun l n v), ngc li, s tham gia ca kiu thc th NVvo kiu lin kt l ton b (bi v n v no cng phi c ngi qun l).

9. Kiu lin kt NHNVIN NHNVIN c t s lc lng1:N (mt nhn vin c th gim st nhiu nhn vin khc). S tham gia ca cakiu thc th NHNVIN ( c hai pha) l b phn (bi v khng phi nhn vinno cng gim st nhn vin khc, v khng phi nhn vin no cng b gim st).Kiu thc th NHNVIN y ng hai vai tr khc nhau: vai tr ngi gimst v vai tr ngi b gim st.

10. Kiu lin kt NHNVIN DN l c t s lc lngl M:N (mt nhn vin c th lm vic trn nhiu d n khc nhau v mi d n cnhiu nhn vin lm vic). S tham gia ca kiu thc th NHNVIN l b phn

(bi v khng phi tt c nhn vin u lm vic trn d n) ngc li, s tham giaca kiu thc th DN l ton b (bi v d n no cng phi c nhn vin lmvic). Kiu lin kt ny c mt thuc tnh l Sgi, ghi li s gi lm vic ca mtnhn vin trn mt d n.

11. Kiu lin kt NHNVIN CON biu th mi lin h gia kiu thcth NHNVIN v kiu thc th CON (mt nhn vin c th c nhng ngicon). Kiu lin kt ny c t s lc lng 1:N (mt nhn vin c th c nhiungi con nhng mi con l con ca ch mt nhn vin). S tham gia ca kiu thc

th NHNVIN l b phn (khng phi nhn vin no cng c con), ngc li, stham gia ca kiu thc th CON l ton b (ngi con no cng phi l con camt nhn vin). Ngoi ra, kiu thc th CON l mt kiu thc th yu.

Sau khi phn tch nh trn, v lc ER ta loi b cc thuc tnh cxem nh cc kiu lin kt ra khi cc kiu thc th. l cc thuc tnh

Ngiqunl v Ngybtu ca kiu thc th NV, thuc tnh nvkimsot

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ca kiu thc th DN, thuc tnh nv v thuc tnh Ngigimst ca kiuthc th NHNVIN, thuc tnh Nhnvin ca kiu thc th PHTHUC .

Kt qu, chng ta c lc ER nh sau:

Hnh II-8. Lc ER CNGTY

NHNVIN NV

CON

DN

Qunl

Lmviccho

Kimsot

Lmvic

trn

Gim st

C

Ms Tn aiHt

n

Ms

H

m

Tn

NgysinhLngGiitnh ach

Ngybtu

Sgi

Tn Ngysinh Giitnh

Tn Ms aim

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IV- M hnh thc th lin kt m rng (m hnh EER)

Mt cch truyn thng, khi xy dng mt c s d liu chng ta thng btu bng vic xy dng m hnh lin kt thc th (m hnh ER) ri sau chuyn i n thnh m hnh quan h. Cc khi nim v m hnh ER c th c

coi l kh y trnh by cc lc c s d liu trong cc ng dng c sd liu truyn thng, ch yu l cc ng dng x l d liu trong kinh doanh vtrong cng nghip. Ngy nay, cc ng dng mi hn cho cng ngh c s d liu tr nn ph bin. Cc c s d liu loi ny i hi nhng yu cu phc tp hnso vi cc ng dng truyn thng. trnh by c cc yu cu ny mt cchchnh xc v r rng, ngi thit k c s d liu phi s dng thm cc khi nimmi. Vic thm vo m hnh ER nhng khi nim mi lm m rng m hnh nyv to nn m hnh ER m rng (gi tt l m hnh EER Enhanced Entity

Relationship Model).M hnh EER bao gm tt c cc khi nim ca m hnh ER, ngoi ra cn c

cc khi nim nh lp, kiu lin kt lp cha/ lp con, tnh tha k, chuyn bit,tng qut, phm tr.

IV.1- Lp cha, lp con v s tha k

Khi nim u tin trong m hnh EER l lp con ca mt kiu thc th. Nhta bit, kiu thc th c s dng biu din c kiu ca thc th v tp hp

cc thc th cng mt kiu trong c s d liu. Trong nhiu trng hp, mt kiuthc th c th c cc nhm con cc thc th ca n v nhng nhm con ny cnc trnh by r rng do ngha ca n i vi c s d liu.

V d, cc thc th thnh vin ca kiu thc th NHNVIN c th cchia thnh cc nhm nh: KS, NGIQUNL, KTHUTVIN Tp ccthc th trong cc nhm l mt tp con ca cc thc th trong tp thc th nhnvin, ngha l mi thc th l thnh vin ca mt trong nhng nhm ny cng l

mt nhn vin. Chng ta gi mi nhm ny l mt lp con ca kiu thc thNHNVIN. Kiu thc th NHNVIN c gi l lp cha ca cc lp con .Ta gi quan h gia lp cha v mt trong nhng lp con ca n l kiu lin kt lpcha/ lp con. Kiu lin kt lp cha/ lp con thng c gi l kiu lin kt l mt(IS_A). Chng ta thng ni rng mt k s l mt nhn vin, mt k thut vin lmt nhn vin.

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Ch rng mt thc th thnh vin trong lp con cng biu din mt thc ththc ti nh mt thnh vin trong lp cha, v vy, cc thc th thnh vin trong lpcon v cc thc th thnh vin trong lp cha l ging nhau, nhng vai tr cachng hon ton khc nhau. Khi chng ta to mt kiu lin kt lp cha/ lp controng h thng c s d liu, chng ta c th trnh by mt thnh vin ca lp connh nh l mt i tng ring bit, mt bn ghi ring bit kt hp vi cc thc thca lp cha ca n qua thuc tnh kho. Kiu lin kt lp cha/ lpcon l mt kiulin kt c t s lc lng 1:1.

Mt khi nim quan trng gn vi cc lp con l s tha k kiu. Kiu camt thc th c xc nh bng cc thuc tnh v cc kiu lin kt m n thamgia. V mi thc th thnh vin trong lp con cng biu din mt thc th thc tinh thc th trong lp cha nn cc gi tr ca thuc tnh ca n trong lp con cng

phi ging nh l gi tr ca cc thuc tnh ca n khi n ng vai tr l mt thnhvin trong lp cha. Thc th ny cng c tha k cc lin kt trong lp cha. Mtlp con vi cc thuc tnh ring ca n cng vi tt c cc thuc tnh v kiu linkt k tha c t lp cha c quyn c coi nh l mt kiu thc th.

IV.2- Chuyn bit ho, tng qut ho

IV.2.1- Chuyn bit ho

L qu trnh xc nh tp hp cc lp con ca mt kiu thc th. Kiu thc thny c gi l lp cha trong chuyn bit ho. Tp cc lp con to nn mt chuyn

bit ho c xc nh da trn c s mt c trng phn bit no ca cc thcth trong lp cha. V d, tp cc lp con {THK, KS, KTHUTVIN} lmt chuyn bit ho ca lp cha NHNVIN c xc nh da trn kiu cngvic ca cc thc th. Mt kiu thc th c th c mt s chuyn bit ho da trncc c trng khc nhau. V d, mt chuyn bit ho khc ca kiu thc th

NHNVIN sinh ra tp cc lp con {NHNVIN_BINCH,NHNVIN_HPNG}. Trong chuyn bit ho ny, cc thc th c phnbit da trn c shnh thc tr tin.

Mt chuyn bit ho c biu din trong s EER nh sau: Cc lp conxc nh mt chuyn bit ho c ni bng cc ng n mt vng trn, vngtrn c ni vi lp cha. K hiu tp con (trn mi ng ni mt tp con vivng trn ch hng ca kiu lin kt lp cha / lp con). Cc thuc tnh ch p dngcho cc thc th ca mt lp con c th - v d nh Tcnhmy ca lp con

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THK - c ni vi hnh ch nht biu din lp con . Cc thuc tnh nh vygi l cc thuc tnh ring hoc l cc thuc tnh a phng ca lp con. Tng t,mt lp con c th tham gia vo cc kiu lin kt ring, v d, lp con

NHNVIN_HPNG tham gia vo kiu lin kt (hnh II-7).

C hai l do chnh t cc kiu lin kt lp cha/lp con v chuyn bit hovo m hnh d liu. Th nht l c mt s thuc tnh c th p dng cho mt scc thc th ch khng phi cho ton b cc thc th ca lp cha. Khi , mt lpcon s c xc nh nhm cc thc th m cc thuc tnh c th p dngc. Cc thnh vin ca lp con ny c th vn chia s phn ln cc thuc tnhca chng vi cc thnh vin khc ca lp cha.V d, lp con THK c thuctnh ring l Tcnhmy, lp con KS c thuc tnh ring l Kiuks nhngcc thuc tnh khc ca chng l chung vi kiu thc th NHNVIN. L do th

hai l ch c cc thnh vin ca lp con c th tham gia vo mt s kiu lin ktno . V d, nu ch c cc nhn vin hp ng mi tham gia v cng on thchng ta c th din t s kin bng cch to ra mt lp con

NHNVIN_HPNG ca NHNVIN v lin kt lp con ny vi kiu thcth CNGON thng qua kiu thc th .

Tm li, qu trnh chuyn bit ho cho php chng ta lm cc vic sau:

Xc nh mt tp hp cc lp con ca mt kiu thc th.

Thit lp cc thuc tnh ring cho mi lp con. Thit lp cc kiu lin kt ring gia mi lp con v cc kiu thc th khc

hoc cc lp con khc.

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Hnh II-9. Biu din lc EER ca chuyn bit ho

IV.2.2-Tng qut ho

L qu trnh o ngc ca chuyn bit ho, trong ta b qua s khc nhaugia mt s kiu thc th, xc nh cc c tnh chung ca chng v tng qut hochng thnh mt lp cha ca cc kiu thc th . V d, ta c kiu thc thXECON vi cc thuc tnh (Ms, Sgiyphp, Gi, Tctia, Schngi) vkiu thc th XETI vi cc thuc tnh (Ms, Sgiyphp, Gi, Trngti,

Scctrc), cc kiu thc th ny c mt s thuc tnh chung, chng c th ctng qut ho thnh kiu thc th XET vi cc thuc tnh (Ms, Sgiyphp,Gi). Cc kiu thc th XECON v XETI tr thnh cc lp con ca lp chaXET. Nh vy, tng qut ho l qu trnh tng qut mt kiu thc th t cckiu thc th cho trc.

NHN VIN

THK KTHUTVIN

K S

d

Ms HtnNgy

sinh

achKiucngvic

Hm Tn

Tcnhmy

KiuK s

Bc

NGIQUNL

d

NHNVINBINCH NHNVINHP NG

Lng LngH

Qunl

D N

Thuc

CNG ON

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Mt tng qut ho c biu din trong s EER ging nh l mt chuynbit ho. Tng qut l lp cha cn chuyn bit l cc lp con c s dng tonn lp cha (hnh II-8).

Hnh II-10. Xet c tng qut ho t Xecon v Xeti

IV.2.3- Phn cp chuyn bit v li chuyn bit

Bn thn cc lp con cng c cc lp con ca n. Nh vy s to ra mt phncp chuyn bit ho hoc mt li chuyn bit ho. Mt phn cp chuyn bit hoc rng buc l mt lp con ch tham gia vo mt kiu lin kt lp cha/lp con nhl mt lp con. Mt li chuyn bitc iu kin l mt lp con c th tham gia

vo nhiu kiu lin kt lpcha/lpcon nh l mt lp con. Ni cch khc, mt lpcon trong phn cp chuyn bit ch tha k mt lp cha, ngc li, mt lp controng li chuyn bit c th tha k nhiu lp cha. Mt lp con tha k nhiu lpcha thuc cc kiu khc nhau c gi l mt kiu hp (union type) hoc mt

phm tr (category).

IV.2.4- Cc rng buc v cc c trung ca chuyn bit ho, tng qutho

Trong mt s chuyn bit ho, chng ta c th xc nh mt cch chnh xccc thc th s l thnh vin ca mt lp con bng cch t mt iu kin trn mtthuc tnh no y ca lp cha. Cc lp con nh vy c gi l cc lp con c

xc nh bng iu kin. Nu cc lp con ca mt chuyn bit ho c iu kinthnh vin trn cng mt thuc tnh ca lp cha th chuyn bit ho cng cgi l chuyn bit ho c xc nh bng thuc tnh. Nu vic xc nh mt lp

Xet

Xe con Xe ti

d

Tc

Sch Strc

Trngti

MsSgiyphp

Gi

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con khng theo mt iu kin no th lp con c gi l c ngi s dngxc nh.

C hai rng buc p dng cho mt chuyn bit ho. Rng buc ri rc ch rarng cc lp con ca mt chuyn bit phi ri rc. iu ny c ngha l mt thc

th c th l mt thnh vin ca nhiu nht l mt trong s cc lp con ca chuynbit ho. Mt chuyn bit ho c xc nh bng thuc tnh tho mn rng bucri rc nu thuc tnh c s dng xc nh thnh vin l n tr. Nu cc lpcon khng tho mn rng buc ri rc, cc tp thc th ca chng c th chngcho nhau, ngha l mt thc th c th l mt thnh vin ca nhiu lp con trongchuyn bit ho. Rng buc th hai trong chuyn bit ho gi l rng buc y ,n c th l ton b hoc tng phn. Mt rng buc chuyn bit ton b ch ra rngmi thc th trong lp cha phi l mt thnh vin ca mt lp con no trong

chuyn bit. Mt rng buc chuyn bit tng phn cho php mt thc th ca lpcha khng thuc v bt k lp con no. V d, nu mt nhn vin phi hoc l mtnhn vin bin ch hoc l mt nhn vin hp ng th (NHNVIN_BINCH,

NHNVIN_HPNG) l mt chuyn bit ton b ca NHNVIN. Nu mtnhn vin c th khng phi l mt th k, mt k s hoc mt k thut vin thchuyn bit (THK, KS, KTHUTVIN) l mt chuyn bit tng phnca NHNVIN.

Trong s ca m hnh EER, nu mt chuyn bit ho l ri rc th gia

hnh trn ni vi cc lp con c ghi ch d (disjoin), cn mt chuyn bit l chngcho th gia hnh trn ni cc lp con c ghi ch o (overlap).

IV.3- S m hnh EER

M hnh EER c biu din ho ging nh m hnh ER, ngha l cc kiuthc th (cc lp) c biu din bng cc hnh ch nht c ghi tn gia, ccthuc tnh ca chng c biu din bng cc hnh van ni vi hnh ch nht.Cc kiu lin kt c biu din bng cc hnh thoi v c ni vi cc kiu thc

th tham gia lin kt. Ti cc hnh thoi c ghi r cc t s lc lng tham gia cacc kiu thc th tham gia vo kiu lin kt. Ngoi ra, kiu lin kt lpcha/lpcon

c biu din bng mt ng ni c thm mt k hiu tp con gia ngni. Cc lp con trong mt chuyn bit c ni vi mt vng trn v vng trnc ni vi lp cha. Nu chuyn bit l ri rc, gia vng trn s ghi ch d, nuchuyn bit l chng cho, gia vng trn c ghi ch o.

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V- Tng kt chng v cu hi n tp

V.1- Tng kt chng

Trong chng ny chng ta tho lun v vai tr ca m hnh d liu bccao trong qu trnh thit k c s d liu. Ta lm quen vi cc khi nim c bnca m hnh lin kt - thc th: kiu thc th, kiu lin kt, v cc thuc tnh cachng. Cc kiu thuc tnh khc nhau cng c xem xt: thuc tnh n, thuctnh phc hp, thuc tnh n tr, thuc tnh a tr, thuc tnh lu tr, thuc tnh suydin c v cc thuc tnh c gi tr null. Thng qua mt v d c th, ta tinhnh xy dng m hnh ER CNGTY. Ngoi ra, chng ta cng ni n mhnh EER, m rng ca m hnh ER. Cc khi nim m rng nh lp, lp con,kiu lin kt lp cha/lp con, chuyn bit ho, tng qut ho cng c gii

thiu v phn tch. Chng ta cng ni n cch biu din ho ca cc mhnh ER v EER.

V.2- Cu hi n tp

1- Hy ni v vai tr ca m hnh d liu bc cao trong qu trnh thit k cs d liu.

2- Lit k cc trng hp cn phi s dng gi tr null.

3- nh ngha cc thut ng sau: thc th, thuc tnh, gi tr thuc tnh, th

hin lin kt, thuc tnh phc hp, thuc tnh a tr, thuc tnh suy din c, thuctnh phc tp, thuc tnh kho, min gi tr.

4- Kiu thc th l g? Tp thc th l g? Gii thch s khc nhau gia mtthc th, mt kiu thc th v mt tp thc th.

5- Gii thch s khc nhau gia mt thuc tnh v mt tp gi tr.

6 - Kiu lin kt l g? Gii thch s khc nhau gia mt th hin lin kt, mttp lin kt v mt kiu lin kt.

7- Vai tr tham gia l g? Khi no cn phi s dng cc tn vai tr trong m tcc kiu lin kt.

8- M t cch ch ra cc rng buc cu trc trn cc kiu lin kt.

9- Vi iu kin no mt thuc tnh ca mt kiu lin kt cp 2 c th chuynthnh mt thuc tnh ca mt trong cc kiu thc th tham gia vo kiu lin kt.

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10- Khi chng ta ngh n cc lin kt nh l cc thuc tnh, cc tp gi tr cacc thuc tnh l g?

11- Kiu lin kt quy l g? Cho mt s v d v cc kiu lin kt quy.

12- Khi no khi nim kiu thc th yu c dng trong m hn ho c s d

liu? nh ngha cc thut ng: kiu thc th ch, kiu thc th yu, kho b phn,kiu lin kt xc nh.

13- Trnh by cc khi nim lp, lp con, chuyn bit ho, tng qut ho.Trong hon cnh no ta cn tch mt lp thnh cc lp con.

14- Trnh by cch biu din ho ca cc m hnh ER v EER.

V.3- Bi tp

Bi 1: Xy dng m hnh ER cho c s d liu TRNGHy xy dng lc ER cho CSDL TRNG, da trn cc ghi chp sau:

1) Trng c chia thnh cc trng con: Trng KHTN, Trng KHXH,Trng Cng ngh,. Mi trng c mt hiu trng qun l. Mi hiutrng qun l mt trng.

2) Mi trng c nhiu khoa. Chng hn, trng KHTN c cc khoa Ton, L,Ho, Mi mt khoa ch thuc v mt trng. Thng tin v Khoa gm M

khoa, tn khoa, a ch, s in thoi, tn trng.3) Mi Khoa cung cp nhiu mn hc. Mi mn hc gm c Tn mn hc, m s,

s n v hc trnh, trnh , tn Khoa.

4) Mi mn hc c th c nhiu hc phn.Mi hc phn c lu gi bng ccthng tin: M hc phn, Tn mn hc, Tn gio vin dy, hc k.

5) Mi khoa c nhiu gio vin lm vic, nhng mi gio vin ch lm vic chomt khoa. Mi mt khoa c mt ch nhim khoa, l mt gio vin.

6) Mi gio vin c th dy nhiu nht l 4 hc phn v cng c th khng dyhc phn no.

7) Mi sinh vin phi hc nhiu hc phn.

8) Mi mt khoa c nhiu sinh vin, mi sinh vin ch thuc v mt khoa. Thngtin v mi sinh vin gm: M sinh vin, H tn, a ch, ngy sinh, gii tnh,Lp, Tn Khoa v ch o to.

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9) Mi sinh vin c mt ngi gim st (gio vin ch nhim), ngi l mtgio vin.

10) Sau mi hc k s c mt danh sch im phn loi. N gm cc thng tin:M sinh vin, m hc phn, im bng ch, im bng s.

Bi 2: Xy dng m hnh ER cho c s d liu TH VIN.

Hy xy dng lc ER cho CSDL TH VIN, da trn cc ghi chpsau:

1) Th vin c chia ra thnh cc nhnh. Thng tin v mi nhnh gm c Mnhnh, Tn nhnh v a ch.

2) Mi cun sch trong th vin c cc thng tin v M sch, Tn sch Nh xutbn v Tc gi

3) Mt tc gi c th vit nhiu cun sch. Mt cun sch c th c nhiu tc givit.

4) Mt nh xut bn xut bn nhiu cun sch. Mt cun sch do mt nh xutbn xut bn. Thng tin v Nh xut bn gm c Tn, ach v Sinthoi.

5) Mt cun sch c th c nhiu bn sao c lu tr ti cc nhnh. Thng tin vbn sao sch gm M sch, s cc bn sao.

6) Th vin c nhng ngi mn sch. Thng tin v nhng ngi mn schgm c S th, H tn, a ch v S in thoi.

7) Sch c cho cc ngi mn mn ti cc nhnh. Thng tin v mt lnmn gm c Ngy mn v ngy tr.

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Chng III- M HNH QUAN H, CC RNG BUCQUAN H V I S QUAN H

M hnh quan h c Ted Codd a ra u tin vo nm 1970 v gy cch ngay tc khc v tnh n gin v cc c s ton hc ca n. M hnh quanh s dng khi nim quan h ton hc nh l khi xy dng c s v c c s lthuyt ca n trong l thuyt tp hp v logic v t bc nht. Trong chng nychng ta s ni v cc c trng c bn ca m hnh, cc rng buc ca chng vtp hp cc php ton ca m hnh quan h.

I- Cc khi nim ca m hnh quan h

M hnh quan h biu th c s d liu nh mt tp cc quan h. Mi quan hc th c biu din nh mt bng gi tr, mi mt dng trong bng biu th mttp hp cc gi tr d liu lin quan vi nhau. Trong chng trc, chng ta ara cc khi nim v kiu thc th v kiu lin kt nh l cc khi nim m hnhho d liu ca th gii thc. Trong m hnh quan h, mi mt dng trong bng

biu th mt s kin tng ng vi mt thc th hoc mt lin kt ca th giithc. Tn bng v tn cc ct dng gip gii thch ngha ca cc gi tr trongmi hng. Mi gi tr trong mt ct u cng mt kiu d liu

Theo thut ng m hnh quan h hnh thc, mi hng c gi l mt b, miu ct c gi l mt thuc tnh, v bng c gi l mt quan h. Kiu d lium t cc kiu ca d liu xut hin trong mi ct gi l mt min

I.1- Min, thuc tnh, b v quan h

Mt min D l mt tp hp cc gi tr nguyn t, iu c ngha l mi gitr trong min l khng th phn chia c trong phm vi m hnh quan h. ct mt min, ngi ta ch ra mt tn, mt kiu d liu v khun dng d liu. Mt

s v d v nh ngha min:

. H tn: Tp hp cc dy ch ci c di


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Ngoi ra, trong c s d liu ngi ta cn ch ra cc thng tin ph th hincc gi tr ca min, chng hn cc n v tnh nh tin, trng lng,

Mt lc quan hR, k hiu l R(A1,A2,..., An), c to nn t mt tnquan h R mt danh sch cc thuc tnh A 1,A2,, An. Mi mt thuc tnh A i l tn

vai tr ca mt min D no trong lc quan h R. D c gi l min gi trca Ai v c k hiu l Dom(Ai). Mt lc quan h c s dng m tmt quan h, R c gi l tn ca quan h . Cp ca mt quan h l s ccthuc tnh ca lc quan h ca n. V d, ta c lc cho quan h cp 5:SINHVIN (Ms, Htn, Ngysinh, Giitnh, ach). Vi lc quan h ny,SINHVIN l tn ca quan h.

Mt quan h(hoc trng thi quan h) r ca lc quan h R(A1,A2,, An)c k hiu l r(R), l tp hp cc n-b r = {t1, t2, ..., tn }. Mi n-b t l mt danh

sch c th t ca n gi tr, t = , trong mi vi ,1


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nh ngha quan h trn c th pht biu li nh sau: Mt quan h r(R) lmt quan h ton hc cp n trn cc min gi tr dom(A1), dom(A2), , dom(An), l tp con ca tch cc ca cc min gi tr xc nh R:

r( R) (dom(A1) x dom(A2) x dom(An))

Tch cc ch ra mi t hp c th c ca cc gi tr t cc min cho. Nhvy, nu ta k hiu lc lng ca mt min D l Dv gi thit rng mi min uhu hn th tng s cc b trong tch cac l:

dom(A1)*dom(A2)*.*dom(An)

Ngoi tt c cc t hp c th c ny, mt trng thi quan h mt thi imcho trc -gi l trng thi quan h hin ti - ch phn nh cc b gi tr biu dinmt trng thi c th ca th gii thc. Ni chung, do trng thi ca th gii thc

thay i, quan h cng b thay i thnh trng thi quan h khc. Tuy nhin, lc R l n nh, khng thay i, tr phi phi thm vo mt s thuc tnh biudin mt thng tin mi cha c lu tr trong quan h.

C th xy ra trng hp nhiu thuc tnh c cng mt min gi tr. Cc thuctnh ch ra cc vai tr khc nhau i vi min. V d, hai thuc tnh achNV vachV c cng min gi tr nhng thuc tnh th nht tham chiu n a chca nhn vin cn a ch th hai tham chiu n a ch ca n v.

I.2- Cc c trng ca cc quan hI.2.1- Th t ca cc b trong mt quan h

Mt quan h c nh ngha nh mt tp hp cc b. Cc phn t trong mttp hp khng c th t, v vy cc b trong mt quan h khng c mt th t cth. Tuy nhin, trong mt tp, cc bn ghi c lu tr mt cch vt l trn a vvy lun c mt th t gia cc bn ghi. Th t ny ch r bn ghi th nht, bnghi th hai, , bn ghi th n. Mt cch tng t, khi ta biu din mt quan h nh

l mt bng, cc hng c hin th theo mt th t nht nh.Th t cc b khng phi l mt phn ca nh ngha quan h bi v mt quan

h c gng biu din cc s vt mc tru tng hoc lgic. C th c nhiu tht lgic trn mt quan h. V d, cc b gi tr trong quan h SINHVIN hnhIII-1 c th sp xp theo nhiu cch khc nhau: theo th t logic ca Htn, theoth t logic ca Ms, nh ngha quan h khng ch ra th t lgic no c, vvy khng c th t lgic no hn th t lgic khc. Cc quan h cha cng mt s

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hng nh nhau nhng cc hng c sp xp khc nhau c xem nh ng nhtvi nhau. Khi mt quan h c ci t nh mt tp, mt th t vt l c th cch ra trn cc bn ghi ca tp.

I.2.2- Th t ca cc gi tr bn trong mt b

Theo nh ngha quan h trn, mt n-b l mt danh sch c th t ca n gitr. Nh vy th t ca cc gi tr trong mt b l quan trng, t suy ra th tca cc thuc tnh trong mt lc quan h cng quan trng. Tuy nhin, mclgic, th t ca cc thuc tnh v cc gi tr ca n l khng thc s quan trngkhi gi c s tng ng gia cc thuc tnh v cc gi tr.

C th a ra mt nh ngha khc v quan h, nh ngha ny s lm cho tht ca cc gi tr trong mt b l khng cn thit. Theo nh ngha ny, mt lc

quan h R = {A1, A2,,An} l mt tp hp cc thuc tnh v mt quan h r(R)l mt tp hp hu hn cc nh x r = {t1, t2, ., tm}, trong mi ti l mt nh x

t R vo D, trong D = dom(A1)dom(A2) dom(An). Trong nh nghany, t[Ai] phi trong dom(Ai) vi 1


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tnh a tr. Cc thuc tnh a tr phi c biu din bng cc quan h cn ccthuc tnh phc hp ch c biu din bng cc thuc tnh thnh phn n ca n.

Cc gi tr ca mt vi thuc tnh trong mt b c th c th khng bit choc khng thch ng cho n. Trng hp , ngi ta s dng mt gi tr c bit

gi l gi tr null.V d, gi s quan h SINHVIN c thuc tnh Sinthoinh.Trong mt tp th sinh vin, c ngi c in thoi nh, c ngi khng c vcng c ngi c nhng khng bit chc. Vi nhng trng hp khng c hockhng bit chc, thuc tnh Sinthoinh c gi tr null.

I.2.4- T h hin ca mt quan h

Mt lc quan h c th c th hin nh l mt tuyn b hoc mtkhng nh. V d lc quan h SINHVIN trn khng nh rng, ni chung,

mt thc th sinh vin c mt m s, h tn, ngy sinh, gii tnh, a ch. Mi btrong quan h c th hin nh l mt s kin hoc nh mt th hin c th camt khng nh. Ngoi cc quan h biu din cc s kin v cc thc th, mt squan h c th biu din cc s kin v mi lin kt. V d, lc quan h

NHNVIN_DN(MsNV, MsDA, Sgi) khng nh cc nhn vin lmvic vi cc d n. Mi b trong quan h ny lin kt mt nhn vin vi mt d nm anh ta lm vic cho n.

Nh vy, m hnh quan h biu din cc s kin v thc th v cc s kin v

lin kt di dng duy nht l cc quan h.

II- Cc rng buc quan h, lc c s d liu quan h

Trong phn ny chng ta tho lun v cc hn ch trn cc d liu trong mtlc c s d liu quan h. Cc hn ch c gi l cc rng buc. C ccloi rng buc: rng buc min, rng buc kho, rng buc ton vn thc th vrng buc ton vn tham chiu.

II.1- Cc rng buc minCc rng buc min ch ra rng gi tr ca mi thuc tnh A phi l mt gi tr

nguyn t thuc min gi tr dom(A). Cc kiu d liu lin kt vi cc min baogm: cc kiu d liu s chun cho cc s nguyn (short integer, integer, longinteger), cc s thc (float, double precision float). Ngoi ra cn cc kiu d liu kt (dy k t vi di c nh, dy k t vi di thay i), ngy, thi gian v

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tin t. Cc loi min khc c th l cc min con ca mt kiu d liu hoc mtkiu d liu m c trong mi gi tr c th c lit k r rng

II.2- Rng buc kho v rng buc trn cc gi tr khng xc nh (null)

Mt quan h c nh ngha nh mt tp hp cc b. Theo nh ngha, cc

phn t ca mt tp hp l khc nhau, v vy, mi b trong quan h phi khcnhau. iu c ngha l khng c hai b c cng mt t hp gi tr cho tt c ccthuc tnh ca chng. Thng thng, c tn ti cc tp con ca cc thuc tnh camt lc quan h c tnh cht l khng c hai b no trong mi trng thiquan h r ca R c cng mt t hp gi tr cho cc thuc tnh ca n. Gi s chngta k hiu mt tp con nh vy l SK, khi vi hai b khc nhau bt k t1 v t2

trong mt trng thi quan h r ca R chng ta c rng buc l t1[SK] t2[SK].

Tp hp thuc tnh SK nh vy c gi l mt siu kho ca lc quanh R. Mt siu kho SK xc nh r mt rng buc v tnh duy nht, pht biu rngkhng c hai b khc nhau trong mt trng thi r ca R c cng mt gi tr cho SK.Mi quan h c t nht l mt siu kho mc nh, l tp hp tt c cc thuctnh ca n. Mt kho Kca mt lc quan h R l mt siu kho ca R vi tnhcht l nu b i bt k thuc tnh A no ra khi K th s cn li mt tp K khng

phi l siu kho ca R. Nh vy, mt kho l mt siu kho ti thiu, ngha l l mt siu kho m ta khng th vt b thuc tnh no ra khi n m vn gi c

rng buc v tnh duy nht.V d, xt quan h SINHVIN vi cc thuc tnh Ms, Htn, Ngysinh,

Giitnh, ach. Thuc tnh {Ms} l mt kho ca SINHVIN bi v khng chai b sinh vin c cng mt gi tr cho Ms. Mi tp hp thuc tnh c chaMs, vd {Ms, Htn, Ngysinh}, u l mt siu kho. Tuy nhin, siu kho{Ms, Htn, Ngysinh} khng phi l kho bi v nu b i thuc tnh Htnhoc Ngysinh hoc c hai th n vn cn l mt siu kho.

Gi tr ca mt thuc tnh kho c th c s dng xc nh mt cch duynht mi b trong mt quan h. V d, gi tr 4515202 ca Ms xc nh mt cchduy nht b gi tr tng ng vi sinh vin L Vn trong quan h SINHVIN. Ch rng mt tp hp thuc tnh to nn mt kho l mt tnh cht ca lc quanh. iu rng buc l tnh cht phi tha mn trn mi trng thi ca lc .Mt kho c xc nh t ngha ca cc thuc tnh v tnh cht l bt bin, tnhcht phi tha mn khi chng ta chn thm cc b mi vo quan h. V d, ta

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khng th v khng c ch nh thuc tnh Htn ca quan h SINHVIN lkho bi v khng c g m bo rng khng tn ti hai sinh vin c cng h tn.

Ni chung, mt lc quan h c th c nhiu hn mt kho. Trong trnghp , mi mt kho c gi l mt kho d tuyn. Thng thng ta phi ch

nh mt trong cc kho d tuyn lm kho chnh ca quan h. Kho chnh l mtkho d tuyn m cc gi tr ca chng c dng xc nh cc b trong quanh. Ta quy c rng, cc thuc tnh to nn kho chnh ca mt lc quan hc gch di. V d:

SINHVIN( Ms, Htn, Ngysinh, Giitnh, ach ).

Ch rng khi mt lc quan h c nhiu kho d tuyn, vic la chnmt kho d tuyn lm kho chnh l tu , tuy nhin tt nht l chn khochnh gm mt thuc tnh hoc c s cc thuc tnh t nht.

Mt rng buc khc trn cc thuc tnh ch r khi no th cho php cc gi trnull. Nhng thuc tnh lun lun phi c mt gi tr xc nh v hp l th b rng

buc l NOT NULL.

II.3- C s d liu quan h v lc c s d liu quan h

trn, chng ta ni n cc lc quan h n l v cc quan h n l.Mt c s d liu quan h thng gm nhiu quan h vi cc b gi tr trong cc

quan h c lin kt vi nhau theo nhiu cch. Trong phn ny chng ta s nhngha mt c s d liu quan h v mt lc c s d liu quan h. Mt lc c s d liu quan h S l mt tp hp cc lc quan h

S = {R1, R2,, Rn} v mt tp cc rng buc ton vn.

Mt trng thi c s d liu quan h (hoc mt c s d liu quan h) DB caS l mt tp hp cc trng thi quan h:

DB = {r1, r2, , rn}

sao cho mi ri l mt trng thi ca Ri v sao cho cc trng thi quan h ri thomn cc rng buc ton vn ch ra trong tp cc rng buc ton vn.

V d, Hnh III-2 trnh by mt lc c s d liu CNGTY v hnh III-3trnh by mt c s d liu cng ty.

NHNVIN(Hm, Tn, MsNV, Ngysinh, ach, Giitnh, Lng,MsNGS, MsV)

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NV(TnV, MsV, MsNQL, Ngybtu)

NV_AIM(MsV, aimV)

DN(TnDA, MsDA, aimDA, M sV)

NHNVIN_DN(MsNV, MsDA, Sgi)

CON(MsNV, TnPT, Giitnh, Ngysinh)

Hnh III-12. Lc c s d liu CNG TY

NHNVIN MsNV Hm Tn Ngy sinh ach Giitnh Lng MsNGS MsV

NV001 L Vn 12/02/79 H ni Nam 3000 NV002 5

NV002 Trnc

Nam 14/02/66 H ni Nam 4000 NV061 5

NV010 Hong Thanh 05/08/79 Nghan

N 2500 NV014 4

NV014 Phm Bng 26/06/52 Bcninh

Nam 4300 NV061 4

NV016 Nguyn Sn 14/08/73 Hnam Nam 3800 NV002 5

NV018 V

Hng

Giang 26/03/83 Nam

nh

N 2500 NV002 5

NV025 Trn L Hoa 15/03/80 Phth

N 2500 NV014 4

NV061 Hong Gip 02/05/47 H tnh Nam 5500 Null 1

NV MsV TnV M sNQL Ngybtdu

5 Nghin cu NV002 15/09/20004 Hnhchnh NV014 24/06/1997

1 Lnho NV061 25/01/1992

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NV_AIM M sV aimV

1 H n

csdl tonghop-sua2

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