On Mon, 2009-06-29 at 17:08 +1000, Robert Edwards wrote: > 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=!# > > Why changing the datatype of coulumn "c"? Will this not probably disable the use of the index? Why not only:
lem=# create table bobtest (a int, b int, c int); CREATE TABLE lem=# create unique index idc_bobtest on bobtest (a, b, coalesce(c, 1)); CREATE INDEX lem=# insert into bobtest (a, b) values (1, 4); INSERT 0 1 lem=# insert into bobtest (a, b) values (1, 4); ERROR: duplicate key value violates unique constraint "idc_bobtest" lem=# insert into bobtest (a, b, c) values (1, 4, null); ERROR: duplicate key value violates unique constraint "idc_bobtest" lem=# > > > > Regards, Andreas > > Beautiful! > > Many thanks, > > Bob Edwards. > -- Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-sql
