Re: Multi-Column Constraints and Null Values

On Sat, Apr 29, 2006 at 13:14:36 -0700, Jeff Frost <jeff@frostconsultingllc.com> wrote:
> On Thu, 27 Apr 2006, Phillip Tornroth wrote:
> 
> >I have a problem in a number of my tables. I'd like to add uniqueness 
> >constraints across multiple columns, but some of the columns are nullable. 
> >This gives me trouble since when I create a constraint on columns A and 
> >B.. I'd like the constraint to be enforced such that you couldn't insert 
> >values ("value for A", null) twice. I understand why the constraints work 
> >this way, but I'm wondering if anyone knows of a workaround.
> >
> 
> Phill,
> 
> You likely want a multicolumn unique index created like so:
> 
> CREATE UNIQUE INDEX foo_A_B_unique_idx ON foo (A,B);
> 
> See the docs here: 
> http://www.postgresql.org/docs/current/interactive/indexes-unique.html

Creating a multicolumn key is a simpler way of doing the same thing.
However, either way of doing the above doesn't completely solve his problem.
To block multiple entries where one of the columns is null he needs to
add extra checks. One way to do this is to add a partial index for each column
with the condition that the other column IS NULL. This will cover everything
but the case where both columns are null. If this case isn't allowed, then
a row constraint can be used to block it. If it is allowed, the only thing that
comes to mind is making an SQL function that counts the number of rows where
both columns are null and calling that function in a row constraint and
checking that the result is <= 1.