Knowledge
I started storing my knowledge in a wiki. At the moment its read only, possibly I will make it writeable later...
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Grouped Subselect
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