Write short notes on Entity Relationship Diagram. BCS051/Assignment

 Ignou Assignment BCS051

Write short notes on Entity Relationship Diagram.

Entity Relationship Diagram (ERD) is a visual representation of underlying data model in the database. It depicts various entities along with its attributes and relationships with other entities and helps visualize the structure of database objects.

Elements of ERD

Entity: All nouns such as person, place, object, eventqualifies as an entity in the ERD. An entity usually has attributes which define its properties. . Examples of entities include Address Details, Person Details, Vehicle details, Student details etc. An entity is depicted by following symbol:

    

Entities are termed as "weak entities" if they depend on other entity for its existence. For instance, an entity such as "Item" which depends on another entity "ItemDetail" is a weak entity. They are denoted by following symbol: 

Attributes describe the properties of an entity. For instance, a Student Entity has attributes such as Studentld, Studentlvame, StudentAddress and StudentPhone. It is denoted by the following symbol:

Key attributes are the ones which uniquely identify the entity. In the above example, StudentId is the key attribute 

Attributes can also be multi-valued if they contain multiple values such as names of countries

Relationship depicts the relationship between two entities and how the data is shared across two entities. Cardinality depicts the number of instances of one entity related to instance of another entity. It can be one-to-one or oneto-many or many-to-many