RE: unique constraint - bug?

>                                 unique (a, b, c)
>                         );
>
>                         insert into letter values('1','2','3');
>                         insert into letter values('1','2','3');
>
>                         insert into letter (a,c) values ('1','3');
>                         insert into letter (a,c) values ('1','3');
>
> Notice that the second insert of ('1','2','3') yields an
> error message, which is the correct behavior - (that is the unique
> constaint is working), while the last insert of ('1','3') does not yield
> an error message - (that is the unique constraint doesn't work in this
> case - I think it should).

NULL != NULL - ie you can't say that both keys are equal.
(That's why PK constraint disallow NULLs...)

Vadim