Обсуждение: Antw: using max() aggregate
Louis-David Mitterrand - ldm@apartia.org - http://www.apartia.fr wrote: > Hello, > > I am trying to return the most recently updated record from a table: > > SELECT max(stopdate) FROM auction; > > and this works but only returns the timestamp, however if I try to get > another column with the aggregate it fails: > > SELECT title,max(stopdate) FROM auction; > ERROR: Attribute auction.title must be GROUPed or used in an aggregate function > > Ok, so I group it now: > > SELECT title,max(stopdate) FROM auction GROUP BY title; > title | max > ---------------+------------------------ > dfsdfsdf | 2000-07-10 05:00:00+02 > dssdfsdfsdfsf | 2000-07-09 16:00:00+02 > sdfsdfsdfsdf | 2001-04-10 15:00:00+02 > (3 rows) > > But the problem is that I now get three rows when I only want the max() > item. > > How should I do it? > > Thanks in advance, I didn't test it, but something like SELECT title,stopdate FROM auction WHERE stopdate = (SELECT max(stopdate) FROM auction); should work. -------- Gerhard
On Fri, Jun 16, 2000 at 09:28:27AM +0200, Gerhard Dieringer wrote: > > I am trying to return the most recently updated record from a table: > > > > SELECT max(stopdate) FROM auction; > > > > and this works but only returns the timestamp, however if I try to get > > another column with the aggregate it fails: > > > > SELECT title,max(stopdate) FROM auction; > > ERROR: Attribute auction.title must be GROUPed or used in an aggregate function > > > > Ok, so I group it now: > > > > SELECT title,max(stopdate) FROM auction GROUP BY title; > > title | max > > ---------------+------------------------ > > dfsdfsdf | 2000-07-10 05:00:00+02 > > dssdfsdfsdfsf | 2000-07-09 16:00:00+02 > > sdfsdfsdfsdf | 2001-04-10 15:00:00+02 > > (3 rows) > > > > But the problem is that I now get three rows when I only want the max() > > item. > > SELECT title,stopdate > FROM auction > WHERE stopdate = (SELECT max(stopdate) FROM auction); > > should work. Thanks for your suggestion. 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). Cheers, -- Louis-David Mitterrand - ldm@apartia.org - http://www.apartia.fr This is Linux Country. On a quiet night you can hear Windows NT reboot.
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 thatviolate 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
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.