The Entity-Relationship Model

Relationships

Relationships are the glue that holds the tables together. They are used to connect related information between tables.

Relationship strength is based on how the primary key of a related entity is defined. A weak, or non-identifying, relationship exists if the primary key of the related entity does not contain a primary key component of the parent entity. Company database examples include:

• Customer(CustID, CustName)
• Order(OrderID, CustID, Date)

A strong, or identifying, relationship exists when the primary key of the related entity contains the primary key component of the parent entity. Examples include:

• Course(CrsCode, DeptCode, Description)
• Class(CrsCode, Section, ClassTime…)