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Entity Relationship Models

Sumant Kulkarni, Yogalakshmi{sumant.kulkarni, j.yogalakshmi}@iiitb.org

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Agenda

Use of ER Model for Conceptual Models

Basic Structure of ER Model

Example of ER model development

Discussion

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ER Model

Concise description of data requirements of the users

For better visualization of entities and relations

Understanding concepts involved in the miniworld

No implementation details

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Basic Elements of ER Model

The miniworld is described in terms of:

Entity

Attribute

Relationships

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Description of an Example of a miniworld

Every student is identified by his student number. The systemalso knows about his Date of Birth, current and permanentaddresses, and his joining date. Every student registers forsome courses or works for projects or does both. Student caneither audit the course or take it for credit. He obtains gradesfor the courses he takes on credit. These grades are availablefor all the subjects the student has taken on credit till now.These credits decide whether he has passed or failed in the

subject.

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Description of an Example of a miniworld

Every course is offered by one or more professors (combined).So, each faculty member spends a specific number of hours onthe course he offers. A professor can be involved in offeringmany courses. Every course belongs to a stream. It also has aname, unique course number. There are some creditassociated with the course.

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Description of an Example of a miniworld

Professor may be in-charges of a lab. A lab can have one ormore in-charges. A lab can have multiple projects runningunder it from specific start dates. If a student opts to work for

a projects, then depending upon the need of the project, hehas to spend some know amount of time on the project.Similar to the student id, there will be unique identificationnumbers for every faculty member, lab and project.

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Elements of ER Model

Entity

Attribute

Composite versus Simple (Atomic) Attributes Single-Valued versus Multivalued Attributes Stored versus Derived Attributes Null Attributes (NA)

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Elements of ER Model

Entity Set

Key Attributes of an Entity Type (uniqueness constraint)

Value Sets (Domains) of Attributes. Value Set of composite attribute.

V = P(V1)XP(V2)X...XP(Vn)where P(Vi) is the powerset of Vi.

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Exploring Relationships

What is Relationship?

Relationship Types Relationship Instances

Relationship Sets

Participation

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Exploring Relationships

Relationship Degree (unary, binary, ternary)

Relationship as Attributes

Role Names and Recursive Relationships

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Constraints on Relationship Types

Cardinality Ratios for Binary Relationships

Total participation (existence dependency) Partial participation

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Attributes of Relationship Types

Migration of Attributes from Relationship Types

Migration in 1:1 Relationship type Migration in 1:N Relationship type.

Migration in M:N Relationship type

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Weak Entity Types

Owner (identifying) entity type

Identifying relationship

A weak entity type always has a total participation constraint.

A weak entity type normally has Partial key

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Summery of Symbols

Figure: Summery of Symbols of E-R modeling

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Student Academic Information System as ER Model

Let us list all the entities from the description given....

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Relations between Entities

Let us List down....

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References

Elmasri, R. and Navathe, S.B. Fundamentals Of DatabaseSystems. Pearson Education

Chen, P.P. The Entity Relationship Model - Towards a UnifiedView of Data. ACM Transactions on Database Systems. 1976

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Any Questions/Inputs/Suggestions?

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