yhdistelmäavaimeen

Yhdistelmäavain, often translated as compound key or composite key in English, is a concept in database design and relational algebra. It refers to a primary key that is composed of two or more attributes (columns) of a table. Instead of a single column uniquely identifying each row, a combination of values from multiple columns is required to guarantee uniqueness.

The purpose of a yhdistelmäavain is to uniquely identify a record when no single attribute is sufficient on its own. This often occurs in tables that represent relationships between other entities, such as a table linking students to courses. A student might enroll in multiple courses, and a course can have multiple students. Neither a student ID nor a course ID alone can uniquely identify an enrollment record. Therefore, a yhdistelmäavain consisting of both student ID and course ID would be used.

When designing a database, choosing the appropriate primary key is crucial for data integrity and efficiency. A yhdistelmäavain can be necessary when there isn't a natural, single-attribute key that can fulfill the uniqueness requirement. However, yhdistelmäavaimet can sometimes lead to more complex queries and are generally less performant than single-attribute primary keys if not managed carefully. They require that all components of the key be specified when referencing a record.