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unique index on fields with possible null values
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* unique index on fields with possible null values
@ 2007-07-27 00:59  Dmitry Ruban <dmitry@ruban.biz>
  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 02:49  Tom Lane <tgl@sss.pgh.pa.us>
  parent: Dmitry Ruban <dmitry@ruban.biz>
  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>
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