Hi,
On Thursday, April 28, 2016 at 11:59:22 PM UTC+2, Rich Rauenzahn wrote:
>
> This is apparently an expected (and standardized) thing in SQL that ('A',
> 'B', NULL) is unique to ('A', 'B', NULL) as NULL is never equal to another
> NULL.
>
Yes, though the standard goes even further: every comparision with NULL
results in false, in that sense NULL is never equal to NULL but also never
unequal to NULL.
> There is a workaround at the SQL level of ...
>
> CREATE UNIQUE INDEX ab_c_null_idx ON my_table (id_A, id_B) WHERE id_C IS NULL;
>
>
This only creates an index on two columns though, you might still be
interested in indexing the cases where A, B, C are not null…
> I'm wondering if this ought to at least be addressed in a runtime warning,
>
Runtime warnings are the worst imo, if at all a system check.
> or at least documentation in unique_together -- and I'm hoping that
> perhaps a Django level workaround could be devised to explicitly ask for
> unique indexes accommodating null values.
>
I am not against a note in the docs, but I find the fact that nulls are not
"unique" and can exist in an index more than once very useful (fwiw
ordering after a column with null can also be interesting across
databases). I'd be interested to hear about your use case -- the "general"
use case is usually that you have an optional column but want to ensure it
is unique as soon as it is filled…
Cheers,
Florian
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