What is a composite key in a relational database?

A composite key in a relational database is a primary key that consists of two or more columns instead of a single column. This combination of columns ensures that each row in a table is uniquely identified, even if none of the individual columns alone can guarantee uniqueness. For example, consider a table storing order details for an e-commerce system. A single order might include multiple products, so using just the order_id column as the primary key wouldn’t work—each order could have multiple entries. However, combining order_id and product_id into a composite key ensures that each product within an order is uniquely identified. Composite keys are particularly useful in scenarios where relationships between entities require multi-column uniqueness constraints.

Composite keys are often used when modeling many-to-many relationships or associative entities. For instance, in a student-course enrollment table, neither the student_id nor the course_id alone can uniquely identify a row, as a student can enroll in multiple courses and a course can have many students. By combining these two columns into a composite key, the database enforces that a student cannot enroll in the same course more than once. Composite keys also align with real-world business rules. In a flight reservation system, a composite key of flight_number, departure_date, and seat_number might ensure no two passengers are assigned the same seat on the same flight. This approach avoids the need for artificial identifiers while maintaining data integrity.

However, composite keys come with trade-offs. They can complicate query writing, especially when joining tables. If another table references a composite key, its foreign key must include all columns of the composite key, which increases complexity. For example, if an invoice table references the order details composite key (order_id, product_id), the invoice table must include both columns in its foreign key constraint. Additionally, composite keys can impact performance, as indexing multiple columns may be less efficient than indexing a single surrogate key (like an auto-incrementing integer). Developers often weigh the benefits of natural composite keys against the simplicity of surrogate keys, choosing based on query patterns, scalability needs, and maintainability. In summary, composite keys are a powerful tool for enforcing multi-column uniqueness but require careful consideration of their implications on database design and usage.