Re: Unique Index

It is clear to me that only allowing a single null value will not
violate the explanation below.

It would be equally true that allowing multiple null values would not
violate it.

-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of Tom Lane
Sent: Thursday, January 20, 2005 8:03 AM
To: Greg Stark
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Unique Index

Greg Stark <gsstark@mit.edu> writes:
> Tom Lane <tgl@sss.pgh.pa.us> writes:
>> Not for UNIQUE constraints.  SQL92 section 4.10 "Integrity
constraints":
>>
>> A unique constraint is satisfied if and only if no two rows in
>> a table have the same non-null values in the unique columns.

> That's ambiguous. Does it mean no two rows have all non-null columns
that are
> all identical? Or does it mean no two rows have columns that excluding
any
> null columns are identical.

OK, try the more formal definition in 8.9 <unique predicate>

         2) If there are no two rows in T such that the value of each
column
            in one row is non-null and is equal to the value of the cor-
            responding column in the other row according to Subclause
8.2,
            "<comparison predicate>", then the result of the <unique
predi-
            cate> is true; otherwise, the result of the <unique
predicate>
            is false.

(11.7 defines the UNIQUE constraint in terms of the unique predicate)

            regards, tom lane

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