Foreign Keys v12

Suppose you want to make sure all employees belong to a valid department. This is called maintaining the referential integrity of your data. In simplistic database systems this would be implemented (if at all) by first looking at the dept table to check if a matching record exists, and then inserting or rejecting the new employee record. This approach has a number of problems and is very inconvenient. Advanced Server can make it easier for you.

A modified version of the emp table presented in Creating a New Table is shown in this section with the addition of a foreign key constraint. The modified emp table looks like the following:

    empno           NUMBER(4) NOT NULL CONSTRAINT emp_pk PRIMARY KEY,
    ename           VARCHAR2(10),
    job             VARCHAR2(9),
    mgr             NUMBER(4),
    hiredate        DATE,
    sal             NUMBER(7,2),
    comm            NUMBER(7,2),
    deptno          NUMBER(2) CONSTRAINT emp_ref_dept_fk
                        REFERENCES dept(deptno)

If an attempt is made to issue the following INSERT command in the sample emp table, the foreign key constraint, emp_ref_dept_fk, ensures that department 50 exists in the dept table. Since it does not, the command is rejected.

INSERT INTO emp VALUES (8000,'JONES','CLERK',7902,'17-AUG-07',1200,NULL,50);

ERROR:  insert or update on table "emp" violates foreign key constraint
DETAIL:  Key (deptno)=(50) is not present in table "dept".

The behavior of foreign keys can be finely tuned to your application. Making correct use of foreign keys will definitely improve the quality of your database applications, so you are strongly encouraged to learn more about them.