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* unique index on fields with possible null values
@ 2007-07-27 00:59 Dmitry Ruban <dmitry@ruban.biz>
2007-07-27 02:49 ` Re: unique index on fields with possible null values Tom Lane <tgl@sss.pgh.pa.us>
0 siblings, 1 reply; 2+ messages in thread
From: Dmitry Ruban @ 2007-07-27 00:59 UTC (permalink / raw)
To: pgsql-sql
Hello pgsql-sql,
I'm trying to find a solution for unique index on fields with possible
null values.
Example table:
CREATE TABLE test (
a integer NOT NULL,
b integer NULL
);
As long as unique index can't check if there are records with null
values i found the only one solution for this problem:
CREATE UNIQUE INDEX test_uniq1 ON test (a, COALESCE(b,0));
Are there any other ways of doing this?
--
Best regards,
Dmitry mailto:dmitry@ruban.biz
^ permalink raw reply [nested|flat] 2+ messages in thread
* Re: unique index on fields with possible null values
2007-07-27 00:59 unique index on fields with possible null values Dmitry Ruban <dmitry@ruban.biz>
@ 2007-07-27 02:49 ` Tom Lane <tgl@sss.pgh.pa.us>
0 siblings, 0 replies; 2+ messages in thread
From: Tom Lane @ 2007-07-27 02:49 UTC (permalink / raw)
To: Dmitry Ruban <dmitry@ruban.biz>; +Cc: pgsql-sql
Dmitry Ruban <dmitry@ruban.biz> writes:
> I'm trying to find a solution for unique index on fields with possible
> null values.
You appear to be hoping that a unique index would constrain a column to
contain at most one null entry. It doesn't work like that, and I
strongly urge you to reconsider what you're using null for. I think you
are in for a world of hurt, well beyond this one particular point,
because the SQL spec's semantics for null are not designed around the
idea that it represents a single distinguishable value. What it
represents is "unknown".
regards, tom lane
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2007-07-27 00:59 unique index on fields with possible null values Dmitry Ruban <dmitry@ruban.biz>
2007-07-27 02:49 ` Tom Lane <tgl@sss.pgh.pa.us>
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