Entity Relationship Diagram or ER Diagram
Entity Relationship Diagram or ER Diagram introduced by Dr. Peter Chen in 1976.
ER diagram consists collection of basic objects called entities.
ER Diagram is a non-technical design method. Basically it is diagrammatic representation of data. It is easy to understand even by non-technical user.
An entity in entity relationship diagram is a thing or an object present in real world that is distinguishable or identify from other objects based on the values of the attributes.
Types of Entity:
1. Tangible:- Entities which is physically exist in real world.
For example: Car, Bank-Locker etc.
2. Intangible:- Entities which is exist logically.
For example: Account number etc.
Entity Set is a collection or set of same type of entities that share same properties or attributes.
It is represent by table in a relational model. However, It is represent by rectangle in entity relationship diagram ER diagram.
Attributes are the units that describe the characteristics of entities.
It is represent by ellipse or oval in entity relationship diagram.
Types of attributes:
1.Simple: Simple attributes can’t be divided further. Therefore, it is represent by simple oval.
2.Composite: Composite attributes can be divided further in simple attributes. This attribute is represent by oval shape which is connect to another oval shape.
3.Single valued: Single valued attribute can have only one value at an instance of time.
For example: Registration number etc.
4.Multi valued: Multi valued attribute can have more than one value at an instance of time.
For example: Phone number etc.
5.Stored: Stored attribute is a value, stored in the database.
For example: Date of Birth (This stored value can be used to derive runtime value).
6.Derived: Derived attribute is a value that can be compute in run time using stored attribute.
For example: Age.
Relationship in entity relationship diagram ER diagram is an association between two or more entities of same or different entity set.
It is represent by diamond shape in entity relationship diagram.
Every relationship has three components:
1.Name (Every relationship have a unique name).
3.Cardinality ratio or participation constraints
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