Re: uniqueness constraint with NULLs

In response to Robert Edwards :
> A. Kretschmer wrote:
> >In response to Robert Edwards :
> >>Can anyone suggest a way that I can impose uniqueness on a and b when
> >>c is NULL?
> >
> >Sure, use a functional index:
> >
> >test=# create table bobtest (a int, b int, c int);
> >CREATE TABLE
> >test=*# create unique index idx_bobtest on
> >bobtest(a,b,coalesce(c::text,'NULL'));
> >CREATE INDEX
> >test=*# insert into bobtest (a, b) values (1, 4);
> >INSERT 0 1
> >test=*# insert into bobtest (a, b, c) values (1, 4, NULL);
> >ERROR: duplicate key value violates unique constraint "idx_bobtest"
> >test=!#
> >
> >
> >Regards, Andreas
>
> Beautiful!

Btw:

if your data doesn't contains 0 or negative, you can use coalesce(c,0)
or coalesce(c,-1). It would be better for searching.

Andreas
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Andreas Kretschmer
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