| Lists: | pgsql-sql |
|---|
| From: | Dmitry Ruban <dmitry(at)ruban(dot)biz> |
|---|---|
| To: | pgsql-sql(at)postgresql(dot)org |
| Subject: | unique index on fields with possible null values |
| Date: | 2007-07-27 00:59:38 |
| Message-ID: | 248746715.20070727125938@ruban.biz |
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| Lists: | 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(at)ruban(dot)biz
| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Dmitry Ruban <dmitry(at)ruban(dot)biz> |
| Cc: | pgsql-sql(at)postgresql(dot)org |
| Subject: | Re: unique index on fields with possible null values |
| Date: | 2007-07-27 02:49:31 |
| Message-ID: | 6521.1185504571@sss.pgh.pa.us |
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| Lists: | pgsql-sql |
Dmitry Ruban <dmitry(at)ruban(dot)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