gistfile1.txt

# Simple version

SELECT
  username,
  max(connected),
  max(disconnected)
FROM radacct
GROUP BY username


# Using subqueries
SELECT
  lc.username,
  lc.last_connect,
  dc.last_disconnect
FROM (
       SELECT
         username,
         max(connected) AS last_connect
       FROM radacct
       GROUP BY username) AS lc LEFT JOIN (SELECT
                                             username,
                                             max(disconnected) AS last_disconnect
                                           FROM radacct

                                           GROUP BY username) AS dc ON dc.username = lc.username
WHERE lc.last_connect <= dc.last_disconnect