Can I group by something that isn't in the SELECT line?

Question

Given a command in SQL;

SELECT ...
    FROM ...
    GROUP BY ...

Can I group by something that isn't in the SELECT line?

Answer 1

Yes.

This is often used in the superaggregate queries like this:

SELECT  AVG(cnt)
FROM    (
        SELECT  COUNT(*) AS cnt
        FROM    sales
        GROUP BY
                product
        HAVING  COUNT(*) > 10
        ) q

, which aggregate the aggregates.