Grouped Subselect

From Flo's Knowledge in a Nutshell

Select daily count of users grouped by day:

SELECT count(*), lastdate.dateadd 
FROM 
users, 
(SELECT (date(date_add)  + integer '1') AS dateadd FROM users GROUP BY dateadd ORDER BY dateadd) AS lastdate 
 WHERE date_add <= lastdate.dateadd GROUP BY dateadd ORDER BY datea