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SELECT
  CASE
    WHEN s.`type` = 'user'
      THEN u.`login`
    WHEN s.`type` = 'reseller'
      THEN r.`login`
    WHEN s.`type` = 'pop3'
      THEN p.`login`
    ELSE ''
  END login
FROM `sessions` s, `users` u, `resellers` r, `pop3` p
WHERE s.`session` = ?
  AND s.`ip` = ?
  AND (
    (
      s.`type` = 'user'
      AND s.`user` = u.`ID`
    )
    OR (
      s.`type` = 'reseller'
      AND s.`user` = r.`ID`
    )
    OR (
      s.`type` = 'pop3'
      AND s.`user` = p.`ID`
    )
  )
GROUP BY login;