Yhdistelmäavain

Yhdistelmäavain, or composite key in English, is a term used in database design. It refers to a primary key that consists of two or more columns. The combination of values in these columns uniquely identifies each record in a table. Unlike a simple primary key which is a single column, a composite key uses multiple columns to ensure uniqueness.

The purpose of a yhdistelmäavain is to provide a unique identifier for a row when no single column can fulfill this role on its own. This is often encountered in many-to-many relationships, where an intermediate or junction table is created. For instance, in a database for a library, a table linking books to borrowers might use a yhdistelmäavain consisting of the `book_id` and `borrower_id` columns. This combination ensures that a specific borrower has borrowed a specific book only once.

When designing a yhdistelmäavain, it's important to ensure that the chosen columns together indeed create a unique identifier for every row. If there are duplicate combinations of values in the constituent columns, the primary key constraint will be violated. Databases enforce this uniqueness. The order of columns in a yhdistelmäavain can also be significant for performance, especially when creating indexes.