Constraint based on existing rows in table

Question

I have the following situation:
a table something like:
create table allow_only_one_in_set
(item_number number,
set_number number,
special_case number(1));
I want to write a constraint that will not allow you to add a row with special_case set to 1 if there already exists a special_case = 1 in that set.
I think a check constraint cannot work because it would require a sub-query. Correct?
Would the way to do this be to use a trigger, and if so how best to pass a message back to the user (already have a special case in this set)?
Thanks,
Jon

Answer 1

You'd want to create a conditional unique constraint.
For this example i've assumed that set number can only have 1 special case with a value of 1, you can change the unique index as needed....
ME_XE?create table allow_only_one_in_set
  2  (
  3     item_number    number,
  4     set_number     number,
  5     special_case   number(1)
  6  )
  7  ;
Table created.
Elapsed: 00:00:00.01
ME_XE?
ME_XE?create unique index allow_only_one_in_set_U_01 on allow_only_one_in_set
  2  (
  3     case when special_case = 1 then set_number else null end
  4  );
Index created.
Elapsed: 00:00:00.00
ME_XE?insert into allow_only_one_in_set values (1, 1, 1);
1 row created.
Elapsed: 00:00:00.00
ME_XE?insert into allow_only_one_in_set values (1, 1, 0);
1 row created.
Elapsed: 00:00:00.00
ME_XE?insert into allow_only_one_in_set values (1, 1, 1);
insert into allow_only_one_in_set values (1, 1, 1)
ERROR at line 1:
ORA-00001: unique constraint (TFORSYTH.ALLOW_ONLY_ONE_IN_SET_U_01) violated
Elapsed: 00:00:00.04
ME_XE?insert into allow_only_one_in_set values (1, 1, 0);
1 row created.
Elapsed: 00:00:00.01
ME_XE?