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 emp records. This approach has a number of
problems and is very inconvenient.
EnterpriseDB
can make it easier for you.
We will be using a modified version of the emp
table in this section in order to demonstrate the creation and use
of foreign key constraints. Our modified emp
table will look something like the following:
The new declaration of the tables would look like this:
CREATE TABLE emp (
empno NUMBER(4),
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_deptno_fk
REFERENCES dept(deptno) );
Now try inserting an invalid record:
INSERT INTO emp VALUES (1234, 'TESTEMP', 'MANAGER', 7902, '20-JAN-85', 1000, NULL, 50);
ERROR: insert or update on table "emp" violates foreign key constraint "emp_ref_deptno_fk"
DETAIL: Key (deptno)=(50) is not present in table "dept".
The behavior of foreign keys can be finely tuned to your
application. We will not go beyond this simple example in this
tutorial, but just refer you to Chapter 4
for more information. 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.