Code:
SQL> create table me(id number primary key);
Table created.
SQL> create table u(id number references me(id));
Table created.
SQL> insert into me values(10);
1 row created.
SQL> insert into me values(20);
1 row created.
SQL> select * from me;
ID
----------
10
20
SQL> insert into u values(10);
1 row created.
SQL> insert into u values(20);
1 row created.
SQL> select * from me;
ID
----------
10
20
SQL> select * from u;
ID
----------
10
20
SQL> Alter table u
2 add constraint fk_id id references me(id) on delete cascade;
add constraint fk_id id references me(id) on delete cascade
*
ERROR at line 2:
ORA-01430: column being added already exists in table
SQL> alter table u modify(id references me(id) on delete cascade);
alter table u modify(id references me(id) on delete cascade)
*
ERROR at line 1:
ORA-02275: such a referential constraint already exists in the tableHelp me friends....