Question

How can I create unique constraint when I need to treat null values as equals.

For

alter table T1 add constraint T1_UN
unique (nvl(C1, ‘ ‘))

i get

 ORA-00904: : invalid identifier

points on nvl

Thanks!

Answer 1

NOT NULL seems like a better idea, but you could make a function-based index unique:

 create unique index idx_t1 on t1 (nvl(C1, ' '));