Ach gondor,
SELECT substring(s.datetime, 1, 10 ) AS myDate
FROM session s, user u
WHERE s.rf = u.id
GROUP BY myDate
SELECT
DATE(datetime) AS myDate -- [link:http://dev.mysql.com/doc/refman/4.1/en/date-and-time-functions.html#id2807156@title=DATE()] wie von dedlfix vorgeschlagen
FROM session s
INNER JOIN user u -- [link:http://aktuell.de.selfhtml.org/artikel/datenbanken/joins/#inner_join@title=explizite Join-Syntax]
ON s.rf = u.id
GROUP BY myDate
wäre doch soviel schöner :-)
Freundliche Grüße
Vinzenz