Introduction for E-R Modeling (DBMS)

An Entity Relationship (ER) model was introduced by P.P. Chen In 1976. An ER model is defined as a conceptual data model that views the the real world as entities and relationships . An ER-Modeling is useful to a database designer in the followings ways:

1. An ER-Modeling maps well to the relational model the constructs used in ER model can be easily transformed into relational tables.
2. An ER-Model can be used by the database designer to communicate the design to the end user.
3. An ER model can be used by as a design plan by the database developer to implement a data model in specific DBMS software.

Basic Terminology Related To ER-Model

Entities - 

Anything which exists and is distinguishable from one another is known as an entity. for example, EMPLOYEES, TABLE, CHALK, BOARD, PEN etc. are some of the entities. please note that an entity is analogues to a table in the relational model,

Entities are of two types:

• Independent entity 
• Dependent entity

An independent entity is one that does not rely on another for identification. While a dependent entity is one that relies on another for identification, please note that an entity occurrence (or instance) is an individual occurrence of an entity. also note that an occurrence is analogous ta a row in the relational table.

Entity Type (or Set) -

It is defined as a set of entities of the some type that share the same properties of attributes. Associative entities (for intersection entities) are the entities used to associate two or more entities in order to reconcile a many-to-many relationship. please remember that the sub-types entities are used in generalization hierarchies to represent a subset of instance of their parent entity (or super type),