Parent key columns

A parent key is either a primary key or a unique key in the parent table of a referential constraint. This key consists of a column or set of columns. The values of a parent key determine the valid values of the foreign key in the constraint.

If every row in a table represents relationships for a unique entity, the table should have one column or a set of columns that provides a unique identifier for the rows of the table. This column (or set of columns) is called the parent key of the table. To ensure that the parent key does not contain duplicate values, you must create a unique index on the column or columns that constitute the parent key. Defining the parent key is called entity integrity, because it requires each entity to have a unique key.

In some cases, using a timestamp as part of the key can be helpful, for example when a table does not have a "natural" unique key or if arrival sequence is the key.

Primary keys for some of the sample tables are:

Table: Key Column
Employee table: EMPNO
Department table: DEPTNO
Project table: PROJNO

Table 1 shows part of the project table which has the primary key column, PROJNO.

Table 1. Part of the project table with the primary key column, PROJNO
PROJNO	PROJNAME	DEPTNO
MA2100	WELD LINE AUTOMATION	D01
MA2110	W L PROGRAMMING	D11

Table 2 shows part of the project activity table, which has a primary key that contains more than one column. The primary key is a composite key, which consists of the PRONNO, ACTNO, and ACSTDATE columns.

Table 2. Part of the Project activities table with a composite primary key
PROJNO	ACTNO	ACSTAFF	ACSTDATE	ACENDATE
AD3100	10	.50	1982-01-01	1982-07-01
AD3110	10	1.00	1982-01-01	1983-01-01
AD3111	60	.50	1982-03-15	1982-04-15