Relationship Mapping in ER Diagram of Databases

Types of Relationship Mapping

Following are the types of Relationship Mapping,
1. One - to - One Relationship
2. One - to - Many Relationship
3. Many - to - One Relationship
4. Many - to - Many Relationship

1. One - to - One Relationship

• In One - to - One Relationship, one entity is related with only one other entity.
• One row in a table is linked with only one row in another table and vice versa.
  For example: A Country can have only one Capital City.

2. One - to - Many Relationship

• In One - to - Many Relationship, one entity is related to many other entities.
• One row in a table A is linked to many rows in a table B, but one row in a table B is linked to only one row in table A.
  For example: One Department has many Employees.

3. Many - to - One Relationship

• In Many - to - One Relationship, many entities can be related with only one other entity.
  For example: No. of Employee works for Department.
• Multiple rows in Employee table is related with only one row in Department table.

4. Many - to - Many Relationship

• In Many - to - Many Relationship, many entities are related with the multiple other entities.
• This relationship is a type of cardinality which refers the relation between two entities.
  For example: Various Books in a Library are issued by many Students.

Participation Constraints

Following are the two types of Participation Constraints,
1. Total Participation
2. Partial Participation

1. Total Participation

• In Total Participation, every entity in the set is involved in some association of the relationship.
• It is indicated by a double line () between entity and relationship.
  For example: Every Department must have a Manager.

2. Partial Participation

• In Partial Participation, not all entities in the set are involved in association of the relationship.
• It is indicated by a single line () between entity and relationship.