Lt5 ERconstraints EER

7/29/2019 Lt5 ERconstraints EER

1/25


2/25


3/25

Outline

Symbols used in ER Diagram Cardinality Of a relationship

ER Constraints

Keys. Single Value constraints.

Foreign-key, or referential-

integrity.Domain constraints.

General constraints. EER concepts


4/25

4

Single-value constraints

Single-value constraintsThe value in a certain context be unique for anattribute. Keys are single-value constraints. AlsoSingle-value constraints can seen in many-onerelationships.

Ex: 1. In students entity set, st_id is a singlevalue constraint.

2. In courses entity set course_no is asingle value constraint.

3. In the employees and depts entity sets, the

relationship heads is many-one. Here foreach entry

in employees, there will be atmost oneassociated

entity(instance) in depts.


5/25

5

Referential integrity constraints

Weak entity: A weak entitycan beidentified uniquely only by considering theprimary key of another (owner) entity. i.e.,a weak entity can not be identified on its

own. Owner entity set and weak entity set must

participate in a one-to-many relationshipset (one owner, many weak entities).

Weak entity setmust have totalparticipation in this identifying relationshipset.


6/25

6

Referential integrityconstraints Example

There must be referential integrityconstraint from weak entity set to thesupporting entity sets.

employee dependenthas

Emp-id

Emp-name Dep-name

gender

relationship

Birth-date


7/25

7

Referential Integrity Contd..

An integrity constraint specifying thatthe value of an attribute in one relationdepends on the value of a primary key

in the same or another relation. i.e., ina 1:m relationship, a value in thematching column on the many side

must correspond to a value in theprimary key for some row in therelation on the one side or be NULL.


8/25

8

Domain constraint

Constraint specifying set of values orrange of values that an attributevalue should lie.

Ex: 1. gender should be either M orF

2. course_no should start with

AAOC, BITS, CS, IS..

3. room_no should lie between 1

to 300


9/25

9

General constraints (assertions)

Constraints/arbitrary assertions thatare specified in the database.

Ex: 1. no_of_courses taken by astudent

in a semester.

2. no_of_sections for a course.


10/25The ER conceptual schema diagram for the


11/25

The ER conceptual schema diagram for the COMPANY database

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

(1,1)

(1,N)

(1,1) (4,N)

(1,N)

(0,N)

(1,1)

(1,1)(3,N)


12/25


13/25

13

Why EER

The ER modeling concepts are notsufficient for representing new databaseapplications, which have more complex

requirements than do the more traditionalapplications.

Additional semantic data modelingconcepts must be used to represent theserequirements as accurately and clearly aspossible.


14/25

EER

EER model concepts includes

All modeling concepts of basic ER

Additional concepts: subclasses/superclasses

specialization/generalization

categories (UNION types)

attribute and relationship inheritance

These are fundamental to conceptual modeling

The additional EER concepts are used to modelapplications more completely and moreaccurately

EER includes some object-oriented concepts,such as inheritance


15/25

EER Modeling

Sub classes, Super classes andInheritance

Specialization and Generalization

Constraints and Characteristics of

Specialization and Generalization

Modeling of UNION Types Using

Categories


16/25

Sub classes and Super classes

EMPLOYE

E

SECRETARY

ENGINEER

TECHNICIAN

SALARIED_EMPLOYEE

HOURLY_EMPLOYEE

Every entity that is a member of one of thesesubgroupings is also an employee

Superclasses: EMPLOYEE

Subclasses: SECRETARY,ENGINEER,

TECHNICIAN,

SALARIED_EMPLOYEE,

HOURLY_EMPLOYEE

An entity type may have additional

meaningful subgroupings of its entities

d

d


17/25

17

Example

SECRETARY

d

TECHNICIAN

ENGINEER

d

SALARIED_EMP

HOURLY_EMP

MANAGER

EMPLOYEE

MANAGES

PROJECT

BELONGS_TO

TRADE_UNION

WORKSDEPARTMENT

EMPLOYEE: WORKS

SECRETARY: WORKSTECHNICIAN: WORKSENGINEER: WORKSMANAGER: WORKS, MANAGESSALARIED_EMP: WORKSHOURLY_EMP: WORKS, BELONGS_TO

TypingSpeed TGrade

EngType

FnameLname SSN

Addr


18/25

18

Why class/subclass relationships andspecializations

Certain attributes may apply to some butnot all entities of the super class. A subclass is defined in order to group the

entities to which these attributes apply.The members of the subclass may still share

the majority of their attributes with the othermembers of the super class.

EMPLOYEE (Name, SSN, BirthDate, Address)SECRETARY (Name, SSN, BirthDate, Address,TypingSpeed)

ENGINEER (Name, SSN, BirthDate, Address,EngineerType)

TECHNICIAN (Name, SSN, BirthDate, Address,TGrade)


19/25

19

Why need class/subclass relationshipsand specializations

Some relationship types may be

participated in only by entities thatare members of the subclass.


20/25

20

Sub classes vs. Super classes

The set of entities in each sub class is a subsetof the entities that belong to EMPLOYEE

Each is called a subclass of EMPLOYEE

EMPLOYEE is the super class for each of thesesubclasses

The relationship between a super class and anyone of its subclass is called a super class/subclass or class/sub class relationship.

e.g., EMPLOYEE/SECRETARY andEMPLOYEE/TECHNICIAN are two class/subclassrelationships.


21/25

Properties of Super classes andSub classes

A member entity of the subclassrepresents the same real-world entityas some member of the super class.

The subclass member is the same as the

entity in the super class, but in a distinctspecific role.

When implementing a super class/subclassrelationship, a member of the subclassmay be represented as a distinctdatabase object a distinct record thatis related via the key attribute to its superclass entity.


22/25

22

Properties of Super classes and Subclasses (cont.)

An entity CANNOT exist in the DB merelyby being a member of a subclass. It mustalso be a member of the super class.

An entity can be a member of more thanone subclass. Example: A salaried employee who is also an engineer belongs

to the two subclasses ENGINEER and SALARIED_EMPLOYEE

It is not necessary that every entity in asuper class be a member of somesubclass Example: A technical writer is an employee but does not

belong to any subclasses.


23/25

23

Type inheritance

The type of an entity is defined by theattributes it possesses and therelationship types which it participates.

An entity that is a member of a subclassinherits all the attributes of the entity asa member of the super class, as well as allthe relationships in which the super classparticipates.


24/25

24

Example

EMPLOYEE

SECRETARYTECHNICIAN

d

Fname

Lname

SSN

TypingSpeed

TGrade

ENGINEER

EngType

SECRETARYFname, Lname, SSN, Addr

TypingSpeedTECHNICIAN

Fname, Lname, SSN, Addr, TGrade

ENGINEERFname, Lname, SSN, Addr,EngType

Addr EMPLOYEEFname, Lname, SSN,Addr


25/25

25

Example

SECRETARY

d

TECHNICIAN

ENGINEER

d

SALARIED_EMP

HOURLY_EMP

MANAGER

EMPLOYEE

MANAGES

PROJECT

BELONGS_TO

TRADE_UNION

WORKSDEPARTMENT

Entity Type: Relationship TypeEMPLOYEE: WORKSSECRETARY: WORKS

TECHNICIAN: WORKSENGINEER: WORKSMANAGER: WORKS, MANAGESSALARIED_EMP: WORKSHOURLY_EMP: WORKS, BELONGS_TO

TypingSpee

d TGradeEngType

Documents

Lt5 ERconstraints EER