A composite key in SQL is a combination of two or more columns in a database table that uniquely identifies each row in that table. Unlike a primary key, which consists of a single column, a composite key ensures uniqueness across multiple columns by treating the combination of those values as a single entity. This is particularly useful in scenarios where a single attribute is not sufficient to guarantee the uniqueness of records, thereby preventing duplicate entries and maintaining data integrity.
For example, consider a table called Orders that tracks orders made by customers. If we have two columns, CustomerID and OrderID, individually they may not be unique, as a customer can place multiple orders. By combining both CustomerID and OrderID as a composite key, we can uniquely identify each order made by a specific customer. This means that while a single CustomerID might appear multiple times in the table, the pairing of it with a specific OrderID ensures that each record remains distinct. Thus, the combination acts as an effective identifier for each order.
Using composite keys also assists in maintaining relationships between tables in a relational database. They are often used in junction tables that relate two other tables in a many-to-many relationship. For instance, in a StudentCourses table that tracks which students are enrolled in which courses, both StudentID and CourseID could act as a composite key together. By doing so, we can easily manage the enrollment records while ensuring that no student is enrolled in the same course multiple times, thus preserving the data's accuracy.