Database constraint check.

Question

Hi I want to add a db check constraint that acts like a unique but its on two columns not one & only i one of the columns is NOT NULL. So if column one is null then i do not want a check but if column one is NOT NULL then check that column one, column two are a unique combination
for example
column 1 column 2
1 1
1 2
1 3
2 1
2 2
null 2
all above are ok.
2 2
this should error as not unique.
Any help much appreciated

Answer 1

Dear,
mhouri> create table t_uk
  2      as
  3      select 1 c1, 1 c2 from dual union all
  4      select 1 c1, 2 c2 from dual union all
  5      select 1 c1, 3 c2 from dual union all
  6      select 2 c1, 1 c2 from dual union all
  7      select 2 c1, 2 c2 from dual;
Table created.
mhouri> alter table t_uk add constraint uk unique (c1, c2);
Table altered.
mhouri> create unique index t_uk_ind on t_uk
  2  (case when c1 is null then 1 else c1 end,
  3   case when c2 is null then 1 else c2 end)   ;
Index created.
mhouri> insert into t_uk values (null,2);
insert into t_uk values (null,2)
ERROR at line 1:
ORA-00001: unique constraint (xxx.T_UK_IND) violated
mhouri> insert into t_uk values (2,null);
insert into t_uk values (2,null)
ERROR at line 1:
ORA-00001: unique constraint (xxx.T_UK_IND) violated
mhouri> insert into t_uk values (2,2);
insert into t_uk values (2,2)
ERROR at line 1:
ORA-00001: unique constraint (xxx.UK) violated
mhouri> insert into t_uk values (2,3);
1 row created.Hope this helps and is correct
Best regards
Mohamed Houri