Composite/Compound key

A composite key consists of two or more columns , designated together as a table's primary key. Multiple-column primary keys can be defined only as table-level constraints:

Single-column primary keys can be defined at either the column or the table level (but not both). For example, the following code states that the table's primary key consists of three columns, JOB_CODE, JOB_GRADE, and JOB_COUNTRY. Neither of these columns is required to be unique by itself, but their combined value must be unique (and NOT NULL).

CREATE TABLE
COLUMN_defs ...
PRIMARY KEY (JOB_CODE,JOB_GRADE,JOB_COUNTRY);

Unfortunately such keys have two huge disadvantages: firstly they slow the database performance considerably, as Firebird/InterBase® needs to check all contents of all columns designated in such a composite key; secondly the sequence of the fields concerned must be identical in all referenced tables.

Basically composite keys should be avoided! It is much preferable to use an internal ID key (so-called artificial key) as the primary key for each table.