relational concepts

8/12/2019 Relational Concepts

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The Relat io n al Mo d el

Chapter 3


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Histo ry o f th e Relat ion al Mod el

The objectives of the relational model :

allow a high degree of data independence, applicationprograms should not be affected by modifications to

the internal data representation, file organisationrecord orderings or access paths

deal with data semantics, consistency and redundancyproblems, the database files should be based on

normalised relationsenable the expansion of set-oriented data manipulationlanguages, using a many-records-at-a-time logic.


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Terminology

RelationA relation is a table with rows and columnsMathematical definition

AttributeAn attribute is a named column of a relation

DomainA domain is the set of allowable values for one ore more

attributes

TupleA tuple is a row of a relation


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The DOMA IN c o nc ept

The domain concept is used to describe a set of possiblevalues for an attribute. This offers more flexibility.

Domain descriptionDomain part-number character 6Domain part-name character 20Domain color character 6Domain weight numeric 4Domain location character 15

Relation PART(P# : Domain part-numberpname : Domain part-namecolor : Domain colorweight : Domain weightcity : Domain location


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Sam p le Database

S S# SNAME STATUS CITYS1 Smith 20 LondonS2 Jones 10 ParisS3 Blake 30 ParisS4 Clark 20 LondonS5 Adams 30 Athens

P P# PNAME COLOR WEIGHT CITY

P1 Nut Red 12 LondonP2 Bolt Green 17 ParisP3 Screw Blue 17 RomeP4 Screw Red 14 LondonP5 Cam Blue 19 LondonP6 Cog Red 19 London

SP S# P# QTYS1 P1 300S1 P2 200S1 P3 400

S1 P5 200S1 P6 100S2 P1 300S2 P2 400

S3 P2 200S4 P2 200S4 P4 300S4 P5 400


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Term ino log y 2

DegreeThe degree of a relation is the number of attributes it contains

CardinalityThe cardinality of a relation is the number of tuples it contains

Relational databaseA collection of normalised relations

Relation SchemaA relation name followed by a set of attribute and domain namepairs


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A lterna tive Term inolo gy

Fo rm al L o g ic al term Clas s ic al

Relation Table File

Tuple Row Record

Attribute Column Field


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Prop ert ies o f a Relat io n

A relation in a database has a unique name.

Each cell of the relation contains exactly one atomicvalue.

Each attribute has a distinct name within a relation.

The values of an attribute are all from the samedomain.

column homogenious

The order of the attributes has no significance.

Each tuple is distinct; there are no duplicate tuples.

The order of tuples has no significance, theoretically.


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Math em atical Relat io n s

Mathematical definition of relation Consider two sets, D 1 and D 2, where D 1 = {2, 4} and D 2

= {1, 3, 5}.

Cartesian product is D 1 D2, the set of all orderedpairs, first element is member of D 1 and secondelement is member of D 2.

Alternative way is to find all combinations ofelements with first from D 1 and second from D 2.

D1 D2 = {(2, 1), (2, 3), (2, 5), (4, 1), (4, 3), (4, 5)}


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Math em atical Relat io n

Any subset of Cartesian product is a relation.For example

R = {(2, 1), (4, 1)}

May specify which pairs are in relation usingsome condition for selection. For example,second element is 1

R = {( x , y ) | x D1, y D2, and y = 1}


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Relat io n al K ey s

Superkey An attribute or a set of attributes that uniquelyidentifies a tuple within a relation.


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Candidate Key

A superkey (K) such that no proper subset is asuperkey within the relation.

In each tuple of R, the values of K uniquely identifythat tuple (uniqueness).

No proper subset of K has the uniqueness property

(irreducibility).


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Primary Key Candidate key selected to identify tuples uniquelywithin relation.

Alternate Keys Candidate keys that are not selected to be theprimary key.


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Foreign Key An attribute or set of attributes within one relationthat matches candidate key of some (possibly same)relation.


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Relat io n al In tegr i ty

Null (value) Represents a value for an attribute that is currentlyunknown or is not applicable for this tuple.Deals with incomplete or exceptional data.Null represents the absence of a value and is not thesame as zero or spaces, which are values.


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Entity IntegrityIn a base relation, no attribute of a primary key canbe null.

Referential IntegrityIf foreign key exists in a relation, either the foreignkey value must match a candidate key value of some

tuple in its home relation or foreign key value mustbe wholly null.


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Enterprise ConstraintsAdditional rules specified by users or databaseadministrators.


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Views

Base relationa named relation, corresponding to an entity in the conceptualschema, whose tuples are physically stored in the database.

Viewa view is the dynamic result of one or more relationaloperations operating on the base relations to produce anotherrelation. A view is a virtual relation that does not actually existin the database but is produced upon request by a particular

user, at the time of request.


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Views

A view is a virtual relation that does not actually existin the database but is produced upon request, at timeof request.

Contents of a view are defined as a query on one ormore base relations.

Views are dynamic, meaning that changes made tobase relations that affect view attributes areimmediately reflected in the view.


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Purp os e of View s

Provides a powerful and flexible securitymechanism by hiding parts of database fromcertain users.

Permits users to access data in a customizedway, so that same data can be seen bydifferent users in different ways, at same time.

It can simplify complex operations on baserelations.


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Updating View s

All updates to a base relation should beimmediately reflected in all views thatreference that base relation.

If view is updated, underlying base relationshould reflect change.


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Updating View s

However, there are restrictions on types ofmodifications that can be made through views:

Updates are allowed if query involves a single baserelation and contains a candidate key of baserelation.Updates are not allowed involving multiple baserelations.Updates are not allowed involving aggregation orgrouping operations.


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Updating View s

Classes of views are defined as theoreticallynot updateable, theoretically updateable andpartially updateable.


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Relat ion al Datab ase A rc h i tectu re

Data Languageapplication

view 1 view 2

Base tableB1

Base tableB2

Base tableB3

Base tableB4

Stored fileS1

Stored fileS2

Stored fileS3

Stored fileS4

External level

Conceptual level

Internal level

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