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Data & Analytics · Intermediate

Composite Key

A key made of two or more columns that together uniquely identify a row.

Definition

When no single column is unique, a combination of columns can serve as the primary or unique key. Composite keys are common in junction tables that resolve many-to-many relationships. Each component may repeat on its own, but the combination must be unique.

Example

PRIMARY KEY (StudentID, CourseID) in an Enrollments table.

Exam tip

Partial dependency (a 2NF violation) can only occur when the primary key is composite.

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