Re: [SQL] Re: [HACKERS] Re: SELECT DISTINCT ON ... ORDER BY ...

Oliver Elphick ha scritto:

> Tom Lane wrote:
> >Thomas Metz <tmetz(at)gsf(dot)de> writes:
> >> SELECT DISTINCT ON id id, name FROM test ORDER BY name;
> >> [doesn't work as expected]
> >
> >There have been related discussions before on pg-hackers mail list;
> >you might care to check the list archives. The conclusion I recall
> >is that it's not real clear how the combination of SELECT DISTINCT
> >on one column and ORDER BY on another *should* work. Postgres'
> >current behavior is clearly wrong IMHO, but there isn't a unique
> >definition of right behavior, because it's not clear which tuples
> >should get selected for the sort.
> >
> >This "SELECT DISTINCT ON attribute" option strikes me as even more
> >bogus. Where did we get that from --- is it in the SQL92 standard?
>
> I looked through the standard yesterday and couldn't find it. It doesn't
> seem to be a useful extension, since it does nothing that you can't do
> with GROUP BY and seems much less well defined. For the moment I have
> added a brief description to the reference documentation for SELECT.
>
> >If you SELECT DISTINCT on a subset of the attributes to be returned,
> >then there's no unique definition of which values get returned in the
> >other columns. In Thomas' example:
> ...
> >Any of these are "DISTINCT ON id", but it's purely a matter of
> >happenstance table order and unspecified implementation choices which
> >one will appear. Do we really have (or want) a statement with
> >inherently undefined behavior?
>
> We have it; I suggest we don't want it!

Yes, seems that SELECT DISTINCT ON is not part of SQL92 but it is very
interesting and I think it is something missing to Standard.
I don't know how to do the following, if we take off DISTINCT ON from
PostgreSQL:

db=> select distinct cognome, nome,via from membri where cap = '41010';
cognome|nome |via
-------+----------------+--------------------------
FIORANI|ELISABETTA |VIA PRETI PARTIGIANI, 63
FIORANI|GASTONE |VIA PRETI PARTIGIANI, 63
FIORANI|MATTIA |VIA PRETI PARTIGIANI, 63
FIORANI|SIMONE |VIA PRETI PARTIGIANI, 63
GOZZI |LILIANA |VIA MAGNAGHI, 39
GOZZI |MATTEO |VIA MAGNAGHI, 39
RUSSO |DAVIDE |STRADA CORLETTO SUD, 194/1
RUSSO |ELENA TERESA |STRADA CORLETTO SUD, 194/1
RUSSO |FORTUNATO |STRADA CORLETTO SUD, 194/1
RUSSO |MAURIZIO ANTONIO|STRADA CORLETTO SUD, 194/1
(10 rows)

db=> select distinct on cognome cognome, nome,via from membri where cap =
'41010';
cognome|nome |via
-------+----------------+--------------------------
FIORANI|GASTONE |VIA PRETI PARTIGIANI, 63
GOZZI |LILIANA |VIA MAGNAGHI, 39
RUSSO |MAURIZIO ANTONIO|STRADA CORLETTO SUD, 194/1
(3 rows)