SELECT a.*
FROM tableX AS a
WHERE a.StatusA <>
( SELECT b.StatusA
FROM tableX AS b
WHERE a.System = b.System
AND a.Timestamp > b.Timestamp
ORDER BY b.Timestamp DESC
LIMIT 1
)
Ale můžete to zkusit také (s indexem na (System,Timestamp)
:
SELECT System, Timestamp, StatusA, StatusB
FROM
( SELECT (@statusPre <> statusA AND @systemPre=System) AS statusChanged
, System, Timestamp, StatusA, StatusB
, @statusPre := StatusA
, @systemPre := System
FROM tableX
, (SELECT @statusPre:=NULL, @systemPre:=NULL) AS d
ORDER BY System
, Timestamp
) AS good
WHERE statusChanged ;