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'-