Re: unique constraint - bug?

Tom Lane wrote:
>Merrill Oveson <merrill(at)actarg(dot)com> writes:
>> It appears as though the null value inserted for column b causes an
>> abrogation of the unique constaint.
>
>Two nulls are never considered equal, therefore the unique constraint
>does not trigger.
>
>This is correct behavior according to SQL92 4.10.2:
>
> 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. In
> ^^^^^^^^
> addition, if the unique constraint was defined with PRIMARY KEY,
> then it requires that none of the values in the specified column or
> columns be the null value.

I think you are not interpreting this right:

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.

I think it means that nulls are to be left out of account in the
comparison. So

a, NULL, c
a, NULL, c

are neither equal nor unequal (as you say) but they do violate a UNIQUE
constraint because the nulls should be ignored altogether. The two
rows do have the "same non-null values".

Surely this interpretation is closer to what users would expect?

--
Oliver Elphick Oliver(dot)Elphick(at)lfix(dot)co(dot)uk
Isle of Wight http://www.lfix.co.uk/oliver
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