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
--
Andreas Kretschmer
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