A composite key in SQL is a combination of two or more columns used to uniquely identify a row in a table. Unlike a single-column primary key, which relies on one field to guarantee uniqueness, a composite key ensures that the combination of values across multiple columns is unique. For example, in a table storing customer orders, a single order might not be uniquely identifiable by just a customer_id or an order_date alone, but the combination of customer_id and order_date could serve as a composite key if it uniquely identifies each order. This approach is useful when no single column in the table can reliably enforce uniqueness on its own.
Composite keys are often employed in scenarios involving relationships between entities, such as junction tables in many-to-many relationships. For instance, consider a table linking students to courses they’ve enrolled in. A student (student_id) can enroll in multiple courses, and a course (course_id) can have multiple students. Neither column alone can uniquely identify a row, but the pair (student_id, course_id) can act as a composite key to ensure no duplicate enrollments. Composite keys also help normalize data by reducing redundancy. Instead of creating a surrogate key (like an auto-incremented integer), using existing columns avoids adding unnecessary fields while maintaining data integrity.
When using composite keys, developers should consider performance and design implications. Indexes created for composite keys can impact query speed, especially if the key columns are frequently used in joins or WHERE clauses. Additionally, foreign keys referencing a composite key must include all columns of the composite key. For example, if a table order_details references the composite key (customer_id, order_date) from an orders table, the order_details table must include both columns in its foreign key definition. While composite keys offer flexibility, they can complicate queries and schema design. Developers should weigh the benefits of natural uniqueness against potential trade-offs in maintainability and performance.