On Fri, Dec 30, 2005 at 13:30:40 -0800,
CSN <cool_screen_name90001@yahoo.com> wrote:
> I have three columns, and one of them can be null. I'd
> like to create a unique constraint across all three
> columns and allow only one null value. e.g.
>
> a|b|c
> abc|123|null
> abc|123|null # not allowed
> abc|456|null
> abc|456|987
> abc|456|876
> def|456|null
> def|456|null # not allowed
>
> Currently, the 'not allowed' lines are allowed.
That is how 'unique' constraints are supposed to work. One possible
solution is to use some normal value instead of 'NULL' to represent
that fact.