Referential integrity

Referential integrity refers to the relationship between tables. Because each table in a database must have a primary key, this primary key can appear in other tables because of its relationship to data within those tables. When a primary key from one table appears in another table, it is called a foreign key.

Foreign keys join tables and establish dependencies between tables. tables can form a hierarchy of dependencies in such a way that if you change or delete a row in one table, you destroy the meaning of rows in other tables. For example, the following figure shows that the customer_num column of the customer table is a primary key for that table and a foreign key in the orders and cust_call tables. Customer number 106, George Watson™, is referenced in both the orders and cust_calls tables. If customer 106 is deleted from the customer table, the link between the three tables and this particular customer is destroyed.
Figure 1: Referential integrity in the demonstration database

This figure is described in the surrounding text.

When you delete a row that contains a primary key or update it with a different primary key, you destroy the meaning of any rows that contain that value as a foreign key. Referential integrity is the logical dependency of a foreign key on a primary key. The integrity of a row that contains a foreign key depends on the integrity of the row that it references—the row that contains the matching primary key.

By default, the database server does not allow you to violate referential integrity and gives you an error message if you attempt to delete rows from the parent table before you delete rows from the child table. You can, however, use the ON DELETE CASCADE option to cause deletes from a parent table to trip deletes on child tables. See The ON DELETE CASCADE option.

To define primary and foreign keys, and the relationship between them, use the CREATE TABLE and ALTER TABLE statements. For more information on these statements, see the HCL OneDB™ Guide to SQL: Syntax. For information about how to build a data model with primary and foreign keys, see the HCL OneDB Database Design and Implementation Guide.