What is a ER Diagram ?
The Entity Relationship diagram defines or describes the relationship among the set of the described objects called entities.
It is graphical representation of the requirements of the database. The ER-D Separates information we need for each entity or group of entity. Thus it will show the overall design of the data base.
Key Features of ER Diagram:
Entity: The entity is the distinguishable or the unique self defined object that has to tracked in the data base. It can be a concrete object like person, abstract like concept, or even the set of self defined entities which can be joint or dis-joint.
Attribute: The attribute describes the characteristics of the entity. Accordingly the attribute can be same to many entities or attribute can be different to the different entities. In such case when we prioritise the characteristics we can easily differentiate the entities.
Eg. In checking the Employee database we can define the Employees as Entities and attribute can be their names and Employee IDs which describes the characteristic called Identification. The Primary Identification of Employee is name. This is the Primary attribute. However the Name can be common to many employees. In such case the Employee Id Number is the distinguishing characteristic and in such case becomes the secondary attribute.
Relationship: Relationship describes how one or more entities interact with one another.
Cardinality : Cardinality is the count if instances that are allowed or necessary between entity relationships. The mapping of cardinality and notation is as follows:
- One-to-one: An entity in A is mutually associated with one entity in B.
- One-to-many: An entity in A is associated with any number in B. However entity in B is associated with at most one entity in A.
- Many-to-one: An entity in A is associated with at most one entity in B. However entity in B is associated with any number in A.
- Many-to-many: Entities in A and B are mutually associated with any number.
Steps to Draw the ER Diagram:
Once the Entities are chosen, we need to choose the respective attributes and also need to prioritize them. Further on mapping the Cardinality as per the Priority and need, we can select the corresponding rows (Attributes) and the columns (Entities) to get the Specified E-R diagram.