Composite Key

Definition

A composite key is a candidate key / primary key made by combining two or more columns (attributes) in a database table to uniquely identify each record (tuple).

What Problem It Solves

Solves the issue where single column is not enough to ensure uniqueness of a row. It enforces entity integrity when data is naturally dependent on multiple attributes.

What Happens If Not Used

Without a composite key, you may face duplicate records, inability to uniquely identify rows, and data integrity issues.

Easy Wording

When one field can’t uniquely identify data, we combine multiple fields to make a unique identity.

Technical Example

In a school, Student ID alone is not enough for exam records. So we combine: StudentID + Subject → unique result entry. Table: ExamResults Columns: StudentID, SubjectID, Marks Composite Key: (StudentID, SubjectID) → Together they uniquely identify each row.