Types of Relationships in ER Diagrams

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Overview

This post summarizes the types of relationships in ER diagrams.

Types of Relationships in ER Diagrams

There are three types of relationships in ER diagrams.

Relationship Type Table Relationship Line Type
Dependent Relationship Child table depends on the parent table (parent-child relationship between tables) Solid line (Parent → Child)
Independent Relationship Child table does not depend on the parent table (no parent-child relationship between tables) Dashed line (Parent → Child)
Many-to-Many Relationship Many-to-many table relationship Solid line (Parent ↔ Child)

Dependent Relationship

  • User

    • UserNo (PK)
    • CompanyNo (FK)
    • Name
    • Email Address

  • User Profile

    • UserNo (FK)
    • Age
    • Gender

The child table, User Profile, cannot exist without a record in the parent table, User. Therefore, the child table is said to depend on the parent table.

Independent Relationship

  • User

    • UserNo (PK)
    • CompanyNo (FK)
    • Name
    • Email Address

  • User Profile

    • UserNo (FK)
    • Age
    • Gender

  • Company

    • CompanyNo (PK)
    • Company Name

As explained earlier, the relationship between User and User Profile is a dependent relationship.

The relationship between User and Company, however, is independent because a company can exist without a user, and vice versa.

Many-to-Many Relationship

  • User

    • UserNo (PK)
    • CompanyNo (FK)
    • PermissionNo (FK)
    • Name
    • Email Address

  • Permission

    • PermissionNo (PK)
    • UserNo (FK)

This type of relationship requires a so-called cross (intermediate or pivot) table.

Thoughts

I want to dive deeper into this topic by reading more books.

References