Entity-relationship(E-R) modelling of data elements of an application.

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Entity-relationship(E-R) modelling of data elements of an application.

LOGICAL DATABASE DESIGN-INTRODUCTION

Loosely one may call organization of related data, put in a table as a “RELATION“. Systematization by which related data are put in a table is called “NORMALIZATION”. A method called entity-relationship analysis facilitates creation of relations.

ENTITY-RELATIONSHIP MODEL

ENTITY: Specifies distinct real world items in an application For example: vendor, item, student, course, teachers RELATIONSHIP: Meaningful dependencies between entities For example: vendor supplies items teacher teaches courses Relationships are underlined above

ENTITY SETS

An entity set is collection of similar entities

Examples :

* Set of all vendors of an organization is a vendor set

* Set of all items in a store is an item set

Every member of an entity set is described by its attributes

ATTRIBUTES

Attributes specify properties of members of entity set & also specify properties of relationships

Examples:

Entity : Vendor

Attributes : vendor code, vendor name, address Relationship : supplies

Attributes : vendor code, item code,order no., qty. supplied,date of supply,price/unit

ENTITES AND ATTRIBUTES

Example

Entity : Teacher

Attributes : Teacher code,teacher name,department,building,room no,phone no.

Relationship : Teaches

Attributes : Teacher code,Course no,course name,semester offered, credits, prerequisites

ENTITY-RELATIONSHIP DIAGRAM

Some entities depend on one another, for example, entity vendor and entity items are related as vendors supply items. These relationships are described by entity-relationship diagrams (or ER diagrams). In an ER diagram entities are represented by rectangles and relationships by diamond shaped boxes

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HOW TO IDENTIFY ENTITIES AND RELATIONSHIPS

When a word statement is used to describe applications, nouns normally are entities and verbs relationships.

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ENTITY-RELATIONSHIP DIAGRAMS

One entity may be related to many other entities by multiple relationships

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Observe a vendor can supply many items, and also that many vendors can supply the same item

RELATION CARDINALITY REPRESENTATION

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WHY IS FINDING CARDINALITY NECESSARY

1. The identifier of the relationship will be composite if cardinality is N:M

2. It will be single if cardinality is 1:M

3. Will be useful later in designing data base

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