Re: counting groups

Holger Klawitter <holger(at)klawitter(dot)de> writes:
> is there any way to count groups in Postgres ? The following
> (simplified) statement should return the number of attributes
> which come up more than once.

> SELECT count(distinct attr)
> FROM table
> WHERE condition
> GROUP BY attr
> HAVING count(*)>1

> But it returns just the right number of 1's.

As it should, since the count() in the SELECT list is evaluated over
each group, and there naturally is going to be only one distinct
value of attr in each group. A single level of SQL query only does
one pass of grouping and aggregation.

In 7.1 it'll be possible to solve this sort of problem by using
table subqueries:

SELECT count(*) FROM
(SELECT attr
FROM table
WHERE condition
GROUP BY attr
HAVING count(*)>1) tab;

which is a little cleaner and a lot more efficient than a WHERE-IN
kind of solution. Doesn't help you much today (unless you're willing to
run development-tip code), but something to keep in mind for later.

BTW, in your proposed workaround

SELECT count(*)
FROM table
WHERE attr IN (
SELECT attr
FROM table
WHERE condition
GROUP BY attr
HAVING count(*)>1
);

I think you still need "SELECT count(distinct attr)" at the outer level.
As is, you'll get the total number of appearances of the attr values
that appear multiple times.

regards, tom lane