Re: unique constraint - bug?

Tom Lane wrote:
  >Merrill Oveson <merrill@actarg.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.Elphick@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
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