Ms. Hatoon Al-Sagri

CCIS – IS Department

The Relational Model and Relational Database Constraints

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Database DesignSteps in building a database for an application:

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Real-world domain

ConceptualModel (EER)

DBMS data model

Create Schema

(DDL)

Load data(DML)

Relational Data Structure

In the relational model, all data is logically structured within relations (tables). Relation is a table with columns & rows. Holds information about entities. Attribute is a named column of a relation. Domain is the set of allowable values for one or more attributes. Every attribute in a relation is defined on a domain. Tuple is a row of a relation. Degree of a relation is the number of attributes it contains. Cardinality of a relation is the number of tuples it contains. Relational database is a collection of normalized relations with distinct relation names.

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Relational Data Structure

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4170010

4182000

4182034

4188134

4189860

4192134

StudentNo

Al-Saleh

Al-Ghanem

Al-Fahad

Saod

Rashed

Al-Fahad

LName

Amal

Nora

Laila

Amal

Rana

Rania

FName Initial

M.

A.

A.

F.

I.

M.

DOB

04-06-78

02-12-79

01-11-74

22-04-73

30-01-78

19-03-79

GPA

3.91

4.20

4.01

3.01

2.31

3.50

STUDENT

Tuples

Attributes

Car

din

ali t

y

Rel

atio

n

Degree

Relation Name

Properties of Relations

The relation has a name that is distinct from all other relation

names in the relational DB

Each cell of the relation contains exactly single value

Each attribute has a distinct name

The values of an attribute are all of the same domain

Each tuple is distinct. There are no duplicate tuples

The order of attributes has no significance

The order of tuples has no significance; theoretically.5

Relational Integrity Constraints

Constraints are conditions that must hold on all valid relation

instances. There are four main types of constraints:

1. Domain constraints

2. Key constraints

3. Entity integrity constraints

4. Referential integrity constraints

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Relational Keys

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Key Constraints

Primary key of R: A set of attributes PK of R such that no

two tuples in any valid relation instance will have the same

value for PK.

Example: The STUDENT relation schema:

STUDENT( Ssn, Name, Dob, Age, Address)

The attribute set {Ssn} is a key of STUDENT because no two student tuples can have the same value for Ssn.

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Entity Integrity Constraints

Entity Integrity: The primary key attributes PK of each relation

schema R in S cannot have null values in any tuple. This is

because primary key values are used to identify the individual

tuples.

PK null for any tuple

 Note: Other attributes of R may be similarly constrained to

disallow null values, even though they are not members of the

primary key.

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DB Relations

Relation schema is a named relation defined by a set of attributes

If A1, A2, .., An are a set of attributes, then relation schema R is:

R = (A1, A2, .., An)

Relational Database Schema is a set of relation schemas, each with a distinct name

If R1, R2, .., Rn are a set of relation schemas, then relational

schema R is:

R= {R1, R2, .., Rn}10

Referential Integrity Constraints

A constraint involving two relations (the previous constraints

involve a single relation).

Used to specify a relationship among tuples in two relations:

the referencing relation and the referenced relation.

Tuples in the referencing relation R1 have attributes FK (called

foreign key attributes) that reference the primary key attributes

PK of the referenced relation R2. A tuple t1 in R1 is said to

reference a tuple t2 in R2 if t1[FK] = t2[PK].

A referential integrity constraint can be displayed in a relational

database schema as a directed arc from R1.FK to R2. 11

Referential integrity constraints displayed on the COMPANY relational database schema.

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Update Operations on Relations

INSERT a tuple.

DELETE a tuple.

MODIFY a tuple.

- Integrity constraints should not be violated by the update

operations.

- Several update operations may have to be grouped together.

- Updates may propagate to cause other updates automatically.

This may be necessary to maintain integrity constraints.13

Update Operations on Relations

In case of integrity violation, several actions can be taken:

– Cancel the operation that causes the violation (REJECT option)

– Perform the operation but inform the user of the violation

– Trigger additional updates so the violation is corrected

(CASCADE option, SET NULL option)

– Execute a user-specified error-correction routine

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