Thread

1. Re: Antw: using max() aggregate

   Gerhard Dieringer <dieringg@eba-haus.de> — 2000-06-16T08:56:04Z

   Louis-David Mitterrand - ldm@apartia.org - http://www.apartia.fr wrote:
   
   > ...
   > Yes this would work nicely but if I need to add more conditional clauses
   > I have to duplicate them in the main SELECT and in the sub-SELECT:
   >
   > SELECT title,max(stopdate)
   > FROM auction
   > WHERE stopdate = (SELECT max(stopdate) FROM auction AND stopdate > now()) 
   > AND stopdate > now();
   >
   > Or am I missing something?
   >
   > Tom Lane suggested using:
   >
   > SELECT title,stopdate FROM auction ORDER BY stopdate LIMIT 1;
   >
   > which seems the best solution (I was a bit concerned about performance,
   > but then again the max() aggregate does a scan of all rows as well).
   > ...
   
   I don't see why you repeat your conditions in the outer select. The condition in the inner select drops all records that violate the conditions,
   so the same conditions in the outer select have nothing to do and you can leave them away.
   
   Tom's solution has the drawback, that if you have more than one record with the same max value you only get one of them, but may be that you want
   to see all of them.
   
   Gerhard
   
   
   
   

2. Re: Antw: using max() aggregate

   Louis-David Mitterrand <cunctator@apartia.ch> — 2000-06-16T10:54:53Z

   On Fri, Jun 16, 2000 at 10:56:04AM +0200, Gerhard Dieringer wrote:
   > > Yes this would work nicely but if I need to add more conditional clauses
   > > I have to duplicate them in the main SELECT and in the sub-SELECT:
   > >
   > > SELECT title,stopdate
   > > FROM auction
   > > WHERE stopdate = (SELECT max(stopdate) FROM auction WHERE stopdate > now()) 
   > > AND stopdate > now();
   > >
   > > Or am I missing something?
   > >
   > > Tom Lane suggested using:
   > >
   > > SELECT title,stopdate FROM auction ORDER BY stopdate LIMIT 1;
   > >
   > > which seems the best solution (I was a bit concerned about performance,
   > > but then again the max() aggregate does a scan of all rows as well).
   > > ...
   > 
   > I don't see why you repeat your conditions in the outer select. The
   > condition in the inner select drops all records that violate the
   > conditions, so the same conditions in the outer select have nothing to
   > do and you can leave them away.
   
   Maybe mine was a bad example but if, for instance, you add a condition
   on the "login" attribute (that it should start with a 'm'), then if you
   omit the clause from the outer select you risk having a false match if
   two records have the same stopdate:
   
   SELECT title,login,stopdate
   FROM auction
   WHERE stopdate = (SELECT max(stopdate) FROM auction WHERE login LIKE 'm%');
   
   > Tom's solution has the drawback, that if you have more than one record
   > with the same max value you only get one of them, but may be that you
   > want to see all of them.
   
   True.
   
   Thanks,
   
   -- 
   Louis-David Mitterrand - ldm@apartia.org - http://www.apartia.fr
   
    Save the whales. Feed the hungry. Free the mallocs.