INSS 651 1

Lecture 3

• Entity Relationship diagram (ERD)

• Cardinalities• Converting ERD into

tables (relations)• An ERD example• Q8/p139

INSS 651 2

Entities

An object of interest• Person• Place• Event• Concept

Ex: StudentOrderCustomer

INSS 651 3

Super & Sub Type Entities

Recognize common theme

WORKER (SUPER TYPE)• PART TIME (SUB TYPE)• FULL TIME (SUB TYPE)• CONTRACTUAL (SUB TYPE)

INSS 651 4

Relationship

• “Has A” between entities

• “IS A” between super & sub entities

TEAM PLAYER

WORKER

PART TIME FULL TIME

INSS 651 5

INHERITANCE

• Sub type entities inherit all attributes & procedures of its SUPER entity type

• Also the basis of Objects

INSS 651 6

Attributes

• Properties of an entity

• Describes an entity

Ex: attribute of an entity STUDENT

student name

student address

etc

INSS 651 7

Domain

Set of allowable values for an attribute

Ex:

Student_age (between 15-85)

Student _classification

(Graduate or

Undergraduate)

INSS 651 8

Relationship

Exists between entities

Recursive ..within itself

Binary between TWO entities

Ternary between THREE entities

INSS 651 9

Nature of Relationship between ENTITIES

• 1:1 (one is to one)

• 1:m (one to many)

• m:n (many to many)

INSS 651 10

Recursive

An entity has relationship to itself, it could be 1:1; 1:m or m:n

EX: EMPLOYEES MARRIED TO OTHER EMPLOYEES

EMPLOYEE

INSS 651 11

CREATE A TABLE WITH EMP_SPOUSE NUMBER ADDED TO IT

EMPLOYEE

(EMP_NO, EMP_NAME,…,SPOUSE_NO)

INSS 651 12

Cardinalities

Relates specific occurrences of ONE entity to other, and vice-versa

MIN cardinalityMAX cardinality

STUDENT COURSE

INSS 651 13

Min, Max

STUDENT <------->CLASS

What’s the minimum number of classes a student can take zero, what’s the max, say 6

What’s the minimum number of students a class can have say 1, max say 40 (class limit), then

STUDENT<------->CLASS

(0,6) (1,40)

INSS 651 14

Another Cardinality Example

Patient Pat_history

What’s the minimum number of patient history a patient can have? 1

What’s the maximum ? Many, say N

What’s the minimum number of patient a patient history can belong to? Maximum?

INSS 651 15

Patient Pat_history

(1, N) (1,1)

INSS 651 16

Converting 1:1; 1:m and m:n into relations

1:1Each entity becomes a relation andPK of either entity becomes a FK in the other entity

ASSUME AN EMPLOYEE CAN have only one car and a car belongs to only ONE employee

EMPLOYEE CAR

Relations:EMPLOYEE (emp_no, emp_name,..,car_id)CAR (car_id, model, make…)Note: or emp_no could be FK in CAR

INSS 651 17

1:m

• PK of PARENT becomes FK in the CHILDEx:Assume: A team can have many players

and a player can play for ONLY ONE team TEAM PLAYER

TEAM (Team_name, location, manager,..)PLAYER (Player_name,…Team_Name)

INSS 651 18

M:N

• Convert m:n into TWO 1:m by introducing an intersection entity

• The intersection contains PK of both entities CLASS STUDENT

CLASS GRADE STUDENT

CLASS (class_id, # of students,..)STUDENT (st_id, st_name, GPA,..)GRADE (class_id, st_id, semester, grade,..)

INSS 651 19

EX: Convert following ERD into relations

assume a student can be advised by one professor but a professor can advise many students, a club can have many students but a student can join only one club, then the ERD will look as

STUDENT

PROFESSOR CLUB

INSS 651 20

Relations will be

STUDENT (St_ID, st_name,…,Club_ID, Prof_ID)

Note: we must account for both relationships in STUDENT entity (student, professor) AND (student, club)

CLUB(Club_ID, location,date_established,..)

PROFESSOR(Prof_ID, Prof_name, specialty,..)

INSS 651 21

Building an ER modelQ8/P139

Identify entities

DINNER

ENTRÉE

GUEST

INSS 651 22

Relationships

• Each dinner is based on a single entrée, but each entrée can be served at many dinners

ENTREE DINNER

INSS 651 23

Cardinality

Cardinality

One entrée can be part of a min of 1 dinner and a max of many dinners, say n

One dinner can be part of a minimum of one entrée and a max of one entrée

(1,N) (1,1)

entree dinner

INSS 651 24

Relationship between DINNER and GUEST

Note there are TWO relationships

A guest can attend many dinners

Vice versa

A guest can have many dinner invitations

Vice versa

invitationDinner Guest

INSS 651 25

Relationship between DINNER and GUEST

Note there are TWO relationships

A guest can attend many dinners

Vice versa

Dinner GuestAttend

INSS 651 26

Relationship between DINNER and GUEST

Second relationship

A guest can have many dinner invitations

Vice versa

Dinner Guestinvitation

INSS 651 27

Converting m:n into two 1:m

since Guest and DINNER is m:n we can break into TWO 1:m

i.e ATTEND entity is intersection of DINNER & GUEST

DINNER ATTEND GUEST

INSS 651 28

cardinality

What is ATTEND entity ? a combination of a dinner and a guest

• What’s the min number of DINNER in ATTEND? 1

• (a dinner must be part of at lest ONE function (ATTEND)

Max number? N (assuming N is total # of dinners)

An attend can be part as a min part of one dinner and a max of ONE dinner also

• what’s the min number of ATTEND a GUEST can have?0

• (a guest may not attend any dinner) or they may attend a max of N dinners

DINNER ATTEND GUEST

(1,N) (1,1) (1,1) (0,N)

INSS 651 29

• Same concepts can be applied to INVITATION intersection entity

INSS 651 30

ERD Q 8/p139

INSS 651 31