Re: counting distinct values

Bruce Momjian wrote:
> 
> > Using the example from
> > http://www.postgresql.org/docs/aw_pgsql_book/node59.html, what would I
> > do if I wanted to know the number of different cities where I had a
> > friend in each state?  select count(city) group by state; would not work
> > because if you had two friends in the same city it would be counted
> > twice.
> >
> 
> How about?
> 
>         select city, state, count(*) group by state, city;

OK that doesn't work for me because I want to use other agregates on the
state level.
Forgetting the state/city example.

playpen=> create table tablem ( a int, b int, n int);
CREATE
playpen=> insert into tablem (a, b, n) values (1, 2, 1);
INSERT 35197 1
playpen=> insert into tablem (a, b, n) values (1, 2, 2);
INSERT 35198 1
playpen=> insert into tablem (a, b, n) values (1, 3, 1);
INSERT 35199 1
playpen=> insert into tablem (a, b, n) values (1, 4, 3);
INSERT 35296 1
playpen=> insert into tablem (a, b, n) values (1, 1, 4);
INSERT 35297 1
playpen=> insert into tablem (a, b, n) values (2, 3, 3);
INSERT 35298 1
playpen=> insert into tablem (a, b, n) values (2, 5, 7);
INSERT 35299 1
playpen=> insert into tablem (a, b, n) values (2, 3, 3);
INSERT 35300 1
playpen=> insert into tablem (a, b, n) values (2, 3, 1);
INSERT 35301 1
playpen=> select a, count(*), avg(n), sum(n) from tablem group by a;
a|count|avg|sum
-+-----+---+---
1|    5|  2| 11
2|    4|  3| 14
(2 rows)


... now suppose I want to have the number of distictive b's in the
results as well. I try:

playpen=> select a, count(m.oid), avg(m.n), sum(m.n), (select
count(mm.oid) from tablem mm where m.a = mm.a group by mm.b ) from
tablem m group by a;
ERROR:  More than one tuple returned by a subselect used as an
expression.
playpen=> 

... even though the subselect should only return one tuple.