Constraint trouble

Question

I have disabled a foreign key constraint and deleted all parent records in table A. I tried to re-enable the constraint and PLSQL complained about children in table B missing parent records. I want to tie the orphaned child records in table B to newly created records in Table C via a separate column. Is there anything I can do to reactivate the FK short of deleting the child records?

Answer 1

You cannot have the same column constrained against two different tables, so you may be out of luck. However, if you need to constraint those orphans against a different column that will be null for the other records, you have two choices.
First, you could insert a dummy parent in the parent table and assign the current orphans to that dummy parent. The other option would be to enable the constraint with the novalidate option.
I assume you situation is something like this:
SQL> CREATE TABLE t (id NUMBER, descr VARCHAR2(10));
Table created.
SQL> CREATE TABLE t1 (t1_id NUMBER, t_id NUMBER, descr VARCHAR2(10));
Table created.
SQL> ALTER TABLE t1 ADD CONSTRAINT t1_t_fk
  2     FOREIGN KEY (t_id) REFERENCES t (id);
Table altered.
SQL> INSERT INTO t VALUES(1, 'One');
1 row created.
SQL> INSERT INTO t VALUES(2, 'Two');
1 row created.
SQL> INSERT INTO t VALUES(3, 'Three');
1 row created.
SQL> INSERT INTO t1 VALUES (1, 1, 'T1One');
1 row created.
SQL> INSERT INTO t1 VALUES (2, 2, 'T1Two');
1 row created.
SQL> INSERT INTO t1 VALUES (3, 3, 'T1Three');
1 row created.
SQL> COMMIT;
Commit complete.
SQL> ALTER TABLE t1 DISABLE CONSTRAINT t1_t_fk;
Table altered.
SQL> DELETE FROM t WHERE id = 3;
1 row deleted.
SQL> COMMIT;
Commit complete.You are here now. Creating a "dummy" parent requires an insert into parent (t in my example) and and update to child (t1 in my example). The novalidate option would go lke:
SQL> ALTER TABLE t1 ENABLE NOVALIDATE CONSTRAINT t1_t_fk;
Table altered.
SQL> INSERT INTO t1 VALUES (4, 3, 'T13Again');
INSERT INTO t1 VALUES (4, 3, 'T13Again')
ERROR at line 1:
ORA-02291: integrity constraint (OPS$ORACLE.T1_T_FK) violated - parent key not foundSo you cannot enter new orphans, but the existing ones will be fine. You can even update them if neccessary:
SQL> UPDATE t1 SET descr = 'Will Fail'
  2  where t1_id = 3;
1 row updated.
SQL> COMMIT;
Commit complete.HTH
John