What is the purpose of a composite key?

A composite key is used in databases to uniquely identify a record when a single key is not sufficient.

In more detail, a composite key is a type of candidate key that is composed of two or more attributes or columns in a table, which together are used to uniquely identify a record. This is particularly useful when a single key is not enough to distinguish between different records.

For instance, consider a university database where the 'Student' table has 'Student_ID' and 'Course_ID' as columns. In this case, 'Student_ID' alone may not be unique as a student can enrol in multiple courses. Similarly, 'Course_ID' alone may not be unique as multiple students can enrol in the same course. Therefore, a composite key combining 'Student_ID' and 'Course_ID' can be used to uniquely identify each record.

Composite keys play a crucial role in maintaining the integrity of data in a relational database. They help to prevent duplicate entries and ensure that each record can be uniquely identified, which is essential for efficient data retrieval and manipulation. Furthermore, they are often used in establishing relationships between tables in a database. For instance, in a many-to-many relationship, a junction table is used with a composite key to link the two main tables.

However, it's important to note that while composite keys can be very useful, they should be used judiciously. Overuse of composite keys can lead to complex queries and can make the database design more complicated. Therefore, they should only be used when necessary to maintain the uniqueness of records.

In conclusion, a composite key is a powerful tool in database design that helps to ensure data integrity and efficient data manipulation. It is particularly useful when a single key is not sufficient to uniquely identify a record.