(SELECT * FROM tA ORDER BY update_time DESC)
UNION
(SELECT * FROM tB ORDER BY update_time DESC)
UNION
(SELECT * FROM tC ORDER BY update_time DESC)
ORDER BY update_time DESC LIMIT 5;
UNION
(SELECT * FROM tB ORDER BY update_time DESC)
UNION
(SELECT * FROM tC ORDER BY update_time DESC)
ORDER BY update_time DESC LIMIT 5;
(SELECT * FROM tA ORDER BY update_time DESC LIMIT 5)
UNION
(SELECT * FROM tB ORDER BY update_time DESC LIMIT 5)
UNION
(SELECT * FROM tC ORDER BY update_time DESC LIMIT 5)
ORDER BY update_time DESC LIMIT 5;