What is a Barred Relationship?

Primary Key Dependency:

In a barred relationship, the primary key of one entity (called the "weak" or "dependent" entity) includes the primary key of another related entity (called the "strong" or "parent" entity).

This means that the dependent entity cannot exist without the parent entity.

Graphical Representation:

In Entity-Relationship (ER) diagrams, a barred relationship is represented by a bar on the line connecting the two entities. This bar indicates that the primary key of the dependent entity includes the primary key of the parent entity.

Practical Example:

In the example you mentioned:

• BANK Entity:

  • Has a primary key: BANK Number.

• ACCOUNT Entity:

  • Has a composite primary key: ACCOUNT Number and BANK Number.

  • This means that the account number (ACCOUNT Number) is only unique within a specific bank (BANK Number).

Why is this important?

• Unique Identification:

  • The primary key of the ACCOUNT entity depends on the BANK Number to be unique. Without the BANK Number, the ACCOUNT Number could repeat across different banks, which would not be suitable.

• Referential Integrity:

  • The barred relationship ensures that an account (ACCOUNT) can only exist if it is associated with a bank (BANK). This maintains data integrity.

In Summary:

A barred relationship occurs when the primary key of one entity includes the primary key of another related entity, indicating a strong dependency between them. In the example, the ACCOUNT entity depends on the BANK entity to form its primary key, and this is represented by a bar on the relationship line.

More:

Identifying (Barred) Relationships

Identifying Relationships with Multiple Entities