How to alter or add foreign key constr to table having child rows or data

Question

Hi,
I have a table by name DEPT in which the column DEPTNO is a primary key column and have data in it for all the columns . Now I have a EMP table where in the column DEPTNO is not having any constraint including foreign key constraint too. Now i have data in the EMP table for all the columns . now i would like to add the foreign key constraint to DEPTNO column in the EMP table referencing the DEPTNO column in the DEPT table. Both the table do not have any null values .
I need to add the foreign key without deleting the data in the EMP table.
Please advice.
Regards,
Vinesh

Answer 1

Hi,
I need to add the foreign key without deleting the data in the EMP table.Foreign key constraint can have only those values which are present in the primary key of the parent table or null.
When your tables are populated with values and then you are adding the constraint then,
the parent table data is only present in child table or null values.
Example
SQL> create table emp_bkp as select * from emp;
Table created.
SQL> create table dept_bkp as select * from dept;
Table created.
SQL> alter table dept_bkp
  2  add constraint pk_deptno primary key(deptno);
Table altered.
SQL> select distinct deptno from emp_bkp;
    DEPTNO
        30
        20
        40
        10
SQL> select deptno from dept_bkp;
    DEPTNO
        10
        20
        30
        40
        55
SQL>  alter table emp_bkp
  2   add constraint fk_deptno_bkp foreign key(deptno)
  3    references dept_bkp(deptno);
Table altered.
SQL> alter table emp_bkp
  2  drop constraint fk_deptno_bkp;
Table altered.
SQL> insert into emp_bkp(empno,ename,deptno) values(142,'Fenny',90);
1 row created.
SQL> alter table emp_bkp
  2  add constraint fk_deptno_bkp foreign key(deptno)
  3   references dept_bkp(deptno);
add constraint fk_deptno_bkp foreign key(deptno)
ERROR at line 2:
ORA-02298: cannot validate (SCOTT.FK_DEPTNO_BKP) - parent keys not found
SQL> update emp_bkp
  2  set deptno=null
  3  where empno=142;
1 row updated.
SQL> alter table emp_bkp
  2  add constraint fk_deptno_bkp foreign key(deptno)
  3   references dept_bkp(deptno);
Table altered.
SQL> insert into dept_bkp (deptno,dname)values(90,'IT');
1 row created.
SQL> update emp_bkp
  2  set deptno=90
  3  where empno=142;
1 row updated.Twinkle