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

Oliver Elphick ha scritto:

> "jose' soares" wrote:
> >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)
>
> This gives the same results:
>
> junk=> select cognome, nome, via from membri where cap = '41010' group by cognome;
> cognome|nome |via
> -------+----------+--------------------------
> FIORANI|ELISABETTA|VIA PRETI PARTIGIANI, 63
> GOZZI |LILIANA |VIA MAGNAGHI, 39
> RUSSO |DAVIDE |STRADA CORLETTO SUD, 194/1

This is very interesting and useful, I thought it wasn't possible. Seems that standard allows
only the "order by" column(s)
and the aggregate function(s) on target list.
I tried the same query on Informix, also on Ocelot but it gives me an error.

On Informix and Ocelot
queries like:
select cognome, max(age) from membri where cap = '41010' group by cognome;
are allowed.
but
queries like:
select cognome, nome, via from membri where cap = '41010' group by cognome;
aren't allowed.

-Jose'-