4.Entity Relationship Model2

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CST203-2 Database Management Systems

Lecture 4


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Student entity

NID FName LName RegNo ExamId Birthdate


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Keys

y AKey is a single attribute or combination of two or

more attributes of an entity set that is used to identifyone or more instances of the set


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Student entity



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Super Key

y Asuper key of an entity set is a set of one or more

attributes whose values uniquely determine eachentity.

y Asuperkey is any set of attributes (K) such that no two

tuples of a relation can have the same values for K, i.e:t1(K) != t2(K) for every possible pair of distinct tuples,t1,t2.


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Student entity



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Candidate Key

y Acandidate key is a superkey such that no proper

subset of K is also a superkey.y Customer-idis candidate key ofcustomer

y account-numberis candidate key ofaccount


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Student entity



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Primary key

y Although several candidate keys may exist, one of the

candidate keys is selected to be theprimary key.

y Primary key does not allow duplicate values(repeating)and null values


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Student entity



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Unique key

y It wont allow duplicate values including NULL...

y Means that it will allow any thing only one time....including null...


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Strong entity types

y Regular entity types which have a primary key

attributes


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Weak Entity Typesy An entity type that does not have a primary key

y The existence of a weak entity type depends on the existence of a

identifying or owner entity typey it must relate to the identifying entity set via a total, one-to-many

relationship set from the identifying to the weak entity sety Identifying relationship depicted using a double diamondy Total participation constraint

y The discriminator(or partialkey) of a weak entity set is the setof attributes that distinguishes among all the entities of a weakentity set.

y Weak entity as an attribute


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y We depict a weak entity set by double rectangles.

y We underline the discriminator of a weak entity set with a dashed line.

y payment-number discriminator of thepayment entity set

yPrimary key forpayment (loan-number, payment-number)


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y In a university, a course is a strong entity and a course-offering can be modeled as a weak entity

y The discriminator ofcourse-offeringwould be semester(including year) and section-number(if there is more thanone section)

yIf we model course-offering as a strong entity we wouldmodel course-numberas an attribute.

Then the relationship with coursewould be implicit in thecourse-numberattribute


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Design of ER diagramInitial design is created and then iteratively refined until the most suitable design

is reached

1. Concept may be first modeled as an attribute and then refined in to

relationship(binary)

2. An attribute that exists in several entity types may be elevated or promotedto an independent entity type

e.g.: student, instructor, course has the common attributedepartment number

3. Inverse of 2nd one : if an entity type exists with a single attribute and isrelated to only one entity .

4. Refine using specialization, generalization and relationships of higherdegree


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Specializationy Top-down design process

y we designate subgroupings within an entity set that are distinctive from other

entities in the set.

y Specific attributes

y Specific relationships

yDepicted by a triangle component labeled ISA(E.g. customeris aperson).

y Attribute inheritance a lower-level entity set inherits all the attributes andrelationship participation of the higher-level entity set to which it is linked


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Insertion and deletion rules for

specialization and generalizationy Deleting from superclass

y Inserting in a superclassy Predicate defined specialization

y Total specialization


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Aggregationy Aclass type really represents a collection of individual

components


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y The system is an aggregation of components

y It denotes an "is part of" relationship


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E-R Diagram for a Banking Enterprise


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Alternative E-R Notations

4.Entity Relationship Model2

Documents

Transcript of 4.Entity Relationship Model2