DBMS Constraints

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Chapter 5

The Relational Data Model andRelational Database Constraints

Copyright 2004 Pearson Education, Inc.


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Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Chapter Outline

hapter 5-3

Relational Model Concepts

Relational Model Constraints and Relational

Database Schemas

Update Operations and Dealing with ConstraintViolations


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Relational Model Concepts

hapter 5-4

The relational Model of Data is based on theconcept of a Relation.

Each relation resembles a table of values Each row in the table represents a collection

of related data values Each row in the table represents a fact tht

typically corresponds to a real world entity orrelationship

A Row is called a tuple

A column header is called a attribute Table is called a relationA Relation is a mathematical concept based

on the ideas of sets.


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FORMAL DEFINITIONS

hapter 5-5

The Schema of a Relation: R(A1, A2, .....An), is madeup of relation name R and list of attributes A1, A2..An

Relation schema Ris defined overattributesA1, A2,.....An

For Example -

CUSTOMER (Cust-id, Cust-name, Address,Phone#)

**Above example is a relation of degree 4

Here, CUSTOMER is a relation defined over the fourattributes Cust-id, Cust-name, Address, Phone#, eachof which has a domain or a set of valid values. Forexample, the domain ofCust-id is 6 digit numbers.


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FORMAL DEFINITIONS

hapter 5-6

A tuple is an ordered set of values Each value is derived from an appropriate domain.

Each row in the CUSTOMER table may be referred to

as a tuple in the table and would consist of four values.

is a tuple belonging to the CUSTOMER relation.

A relation may be regarded as a set of tuples(rows).

Columns in a table are also called attributes of the

relation.


7/23Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

FORMAL DEFINITIONS

hapter 5-7

A domain has a logical definition:

A domain may have a data-type or a format defined forit.

USA_phone_numbers are the set of 10 digit phonenumbers valid in the U.S.

The USA_phone_numbers may have a format: (ddd)-ddd-dddd where each d is a decimal digit.

E.g., Dates have various formats such as monthname,date, year or yyyy-mm-dd, or dd mm,yyyy etc.


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FORMAL DEFINITIONS

hapter 5-8

The relation is formed over the cartesian product of the sets;each set has values from a domain

Cartesian product specifies all possible combinations ofvalues from underlying domain

For example, attribute Cust-name is defined over the domainof strings of 25 characters. The role these strings play in theCUSTOMER relation is that of the name of customers.

R: schema of the relation

r of R: a specific "value" or population of R.

R is also called the intension of a relation

r is also called the extension of a relation


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FORMAL DEFINITIONS

hapter 5-9

Let S1 = {0,1} Let S2 = {a,b,c}

Let R S1 X S2

Then for example: r(R) = { , , }

is one possible state or population orextension r of the relation R, defined over

domains S1 and S2. It has three tuples.


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DEFINITION SUMMARYInformal Terms Formal Terms

Table Relation

Column Attribute/Domain

Row Tuple

Values in a column Domain

Table Definition Schema of a Relation

Populated Table Extension

apter 5-10


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Example - Figure 5.1

hapter 5-11


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CHARACTERISTICS OF RELATIONS

Ordering of tuples in a relation r(R): A relation is defined as aset of tuples. Mathematically elements of a set have no orderamong them hence tuples in a relation do not have any particularorder.

Ordering of attributes in a relation schema R (and of valueswithin each tuple): We will consider the attributes in R(A1, A2, ...,

An) and the values in t= to be ordered.

(However, a more general alternative definition of relation doesnot require this ordering).

Values in a tuple: All values are considered atomic(indivisible). A special null value is used to represent values that

are unknown or inapplicable to certain tuples.


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Relational Integrity Constraints

apter 5-13

Constraints are conditions that must hold on allvalid relation instances. The main types of

constraints are:

1. Domain constraints

2. Key constraints

3. Entity integrity constraints

4. Referential integrity constraints


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Domain Constraints

apter 5-14

Domain constraints specify tht the value of eachattribute A must be an atomic value from the

domain dom(A)

The data types associated with domains typically

include integer, float and character also date, time

and money data types.

Eg- roll no 1 to 50 cant have 60 roll no


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Key Constraints Superkey of R: A set of attributes SK of R such that no

two tuples in any valid relation instance r(R) will havethe same value for SK. That is, for any distinct tuples t1and t2 in r(R), t1[SK] t2[SK].

Key of R: A "minimal" superkey; that is, a superkey Ksuch that removal of any attribute from K results in a set

of attributes that is not a superkey.Example: The CAR relation schema:CAR(State, Reg#, SerialNo, Make, Model, Year)has two keys Key1 = {State, Reg#}, Key2 = {SerialNo}, which are

also superkeys. {SerialNo, Make} is a superkey but not akey.

If a relation has several candidate keys, one is chosenarbitrarily to be the primary key. The primary keyattributes are underlined.

Eg House front door and backdoor key.

Eg- pan no, emp id. To calculate slary we need emp id

not pan no


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Key Constraints

apter 5-16

5.4


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Entity Integrity

apter 5-17

Entity Integrity: The primary key attributes of a tuplecan never be null

t[PK] null for any tuple t in r(R) Note: Other attributes of R may be similarly

constrained to disallow null values, even though theyare not members of the primary key.


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Referential Integrity

apter 5-18

A constraint involving two relations (the previousconstraints involve a single relation).

Used to specify a relationship among tuples in tworelations: the referencing relation and the referencedrelation.

Tuples in the referencing relation R1 have attributes FK(called foreign key attributes) that reference the primarykey attributes PK of the referenced relation R2. A tuple t1 inR1 is said to reference a tuple t2 in R2 if t1[FK] = t2[PK].

In any given relation a set of attributes is called a foreign iffollowing rules hold

The attribute in the foreign key has to be the same as theprimary key in other relation

For every tuple in the referencing relation like dpt theattributes in its foreign key refer to existing tuples like eachdpt shd hv a manager who exist in the database or it shd

be null


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Referential IntegrityConstraint

apter 5-19

Statement of the constraintThe value in the foreign key column (or columns)

FK of the the referencing relation R1 can beeither:

(1) a value of an existing primary key valueof the corresponding primary key PK in thereferenced relation R2,, or..

(2) a null.

In case (2), the FK in R1 should not be a part ofits own primary key.


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Other Types of Constraints

apter 5-20

Semantic Integrity Constraints:

- based on application semantics and cannot beexpressed by the model per se

-E.g., the max. no. of hours per employee for allprojects he or she works on is 56 hrs per week

- A constraint specification languagemay have tobe used to express these

- SQL-99 allows triggers and ASSERTIONS toallow for some of these


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Elmasri/Navathe, Fundamentals of Database Systems, Fourth Editionapter 5-21

5.5


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5.6


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5.7

DBMS Constraints

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