Ghani DBMS _Entity Relationship Model

8/7/2019 Ghani DBMS _Entity Relationship Model

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Entity Relationship Model

Abdul Ghani Khan, M.Tech (ICT)

School of ICT, Guatam Buddha University Page 1


Conceptual Design is an important phase in designing a successful database application

Without conceptual models, it is very difficult to communicate database designs to

(nontechnical) users - This lack of communication may result in users data requirementsbeing missed or incorrect requirements being captured

The Entity Relationship (ER) Model is one of the most widely used method for conceptualdesign.

The ER Model can be neatly mapped to a Relational SchemaModule 3 Contents:

The Entity-Relationship Model

Conceptual Design and how conceptual design relates to the database design process Entity-Relationship (ER) Conceptual Design Method Notation Guide Example Illustrations

Phases of Database Design:

First step of the design process is to identify the Universe of Discourse (UoD) The database to be built will not model everything in the world, but rather some mini-world

or Universe of Discourse.

The UoD is the relevant portion of the real world to be modeled by the database


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Module 3 Contents:

The Entity-Relationship Model

Conceptual Design and how conceptual design relates to the database design process Entity-Relationship (ER) Conceptual Design Method Notation Guide Example Illustrations


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ERModel Basics:

Entities Entities, Entity Sets and Entity Types

Attributes, Keys and Value Sets

RelationshipsRelationship Types and Sets

Relationship Degree

Roles andRecursive Relationships

Relationship Constraints

Attributes ofRelationship Types

Entities:

Entity Entity Sets Entity Type Attributes (Types, Keys and Value Sets)

Real-world object distinguishable from other objects

(e.g a student, car, job, subject, building ...)

An entity is described using a set of attributes The same entity may have different prominence in different UoDs

In the Company database, an employees car is of


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lesser importance

In the Department of Transportations registration

database, cars may be the most important concept

In both cases, cars will be represented as entities;

but with different levels of detail

Entity Sets:

A collection of similar entities (e.g. all employees)

All entities in an entity set have the same set of attributes Each entity set has a key Each attribute has a domain Can map entity set to a relation easily

Entity Type:


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Attributes:

Value Sets ofAttributes:

Value sets specify the set of values that may be assigned to a particular attribute of an entity Employee Age: Integers between 21 & 65

Vehicle Registration Number: String of 3

alphabets followed by 3 integers

Value sets map to relational domains Value sets are not displayed on the ER diagram

Null Valued Attributes:

A particular entity may not have an applicable value for an attribute Tertiary-Degree: Not applicable for a person

with no university education

Home-Phone: Not known if it exists

Height: Not known at present time


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Type of Null Values Not Applicable

Unknown

Missing

Composite Vs. Simple Attributes:


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ER Model Basics:

Entities Entities, Entity Sets and Entity Types

Attributes, Keys and Value Sets

Relationships Relationship Types and Sets

Relationship Degree

Roles and Recursive Relationships

Relationship Constraints

Attributes of Relationship Types


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Relationship Constraints:

What are Relationship Constraints ?

Constraints on relationships are determined by the UoD, which these relationships aredescribing Constraints on the relationship type limit the possible combination of entities that may

participate in the corresponding relationship set

Kinds of Constraints:

What kind of constraints can be defined in the ER Model?

Cardinality Constraints Participation Constraints

Together called Structural Constraints

Constraints are represented by

specific notation in the ER diagram


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Example Cardinality Constraints:

How many Employees can work in a Department?

One employee can work in only one department

How many Employees can be employed by a Department?

One department can employ many employees

How many managers can a department have?

One department can have only one manager

How many departments can an employee manage?

One employee can have manage only one department


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Kinds of participating constraints:

TOTAL Participation (Existence Dependency) Constraint : Every employee must work for a department

PARTIAL Participation Constraint : Not every employee is a manager


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WeakEntities:

Owner and weak entity must participate in a 1:N relationship type Weak entity must have total participation in the identifying relationship set (existence

dependency)

Weak entities normally have a Partial Key, which is a set of attributes that uniquely identifyweak entities related to the same owner entity

Not every existence dependency results in a weak entity !


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Example Illustrations:

Project Management Course Administration Recruitment Company Database

Note how different (interpretations of) semantics change the ER Diagram


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Company Schema (Relational):

EMPLOYEE[Ssn, Fname, Minit, Lname, Bdate, Address, Sex, Salary , SuperSsn ] DEPARTMENT[ Dnumber, Dname MGRSSN, MgrStart ] PROJECT[Pno, PName, Plocation, Dnum ] DEPENDENT[ ESSN,DepName, Sex, BirthDate, Relationship ] WORKS_ON[ ESSN, PNo, Hours ] DEPT_LOCS[ DNumber, Dlocation ]

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