In what cases is a complex primary key consisting of multiple fields used?

A complex primary key consisting of multiple fields (composite key) is used when the uniqueness of a record in a table is determined not by a single field, but by a combination of several. This is useful when:

• There is no single unique field, but there is a unique combination of several.
• It is necessary to guarantee uniqueness across multiple attributes simultaneously.
• In linking tables (for example, to implement many-to-many relationships), the primary key often consists of foreign keys to related tables.

Example: an orders table may have a composite key of order_id and product_id, if one order can contain multiple products, and the uniqueness is defined by this pair.