Description of the illustration cncpt056.eps

This graphic shows the departments table. The department_id column is labeled "Parent Key: Primary key of referenced table." Below it is the employees table labeled "Referenced or Parent Table." The department_id column points to the departments.department_id column. The employees.department_id column is labeled "Foreign Key (values in dependent table must match a value in unique key or primary key of referenced table)." Below the employees table is an insert of two rows. The first row contains 99 as a department ID. The callout says: "This row violates the referential constraint because '99' is not present in the referenced table's primary key; therefore, the row is not allowed in the table." The second inserted row has a null for department_id. The callout says "This row is allowed in the table because a null value is entered in the department_id column; however, if a not null constraint is also defined for this column, this row is not allowed."