What Is an Identified Relationship?

In this relationship, the child entity's primary key includes the parent's primary key, establishing a tight dependency. Identifying relationships is commonly used when modeling weak entities that rely on a strong parent for context and identification. This structure ensures referential integrity and helps represent real-world dependencies clearly in relational databases.

Characteristics of Identifying Relationships

Identifying relationships are used when a child entity depends entirely on a parent entity for its existence. 

These relationships define strong data dependencies and help maintain integrity in relational database design.

• The child is a weak entity: The child entity cannot exist independently and relies on its parent for support.
• Foreign key as part of primary key: The parent’s key is included in the child’s primary key to form a composite key.
• Total participation: The child must be linked to a parent record, indicating full dependency.
• Cascading deletions: Deleting a parent record typically removes all related child records automatically.

Identifying vs. Non-Identifying Relationships: Key Differences

The key difference between identifying and non-identifying relationships lies in the degree of dependency the child entity has on the parent. In an identifying relationship, the child cannot exist independently and includes the parent's primary key in its own. This creates a strong connection and enforces strict referential integrity.

In contrast, a non-identifying relationship allows the child to exist independently of the parent. The parent’s primary key does not form part of the child’s key; only a foreign key reference is maintained. Non-identifying relationships are better suited for loosely connected entities where dependency is not mandatory for existence.

Examples of Identifying Relationships

Identifying relationships is often used when modeling weak entities that rely on a parent for identification.

For example:

• Order and OrderItem: An OrderItem must belong to an Order. Its primary key includes both Order ID and Item ID.
• Department and Employee: If employees are uniquely identified only within a department, the Employee table’s primary key might include DepartmentID.
• Invoice and InvoiceLine: An invoice line cannot exist without its parent invoice; thus, their relationship is identifying.

These relationships clarify dependency and ensure that child records are always tied to a valid parent.

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