Raggruppare dati su un unica colonna

[optime]

con una UNION dovresti cavartela

SELECT * FROM
(
SELECT PicketID_1 AS Employee, Data FROM Tabella WHERE PicketID_1>0
UNION ALL
SELECT PicketID_2 AS Employee, Data FROM Tabella WHERE PicketID_2>0
UNION ALL
SELECT PicketID_3 AS Employee, Data FROM Tabella WHERE PicketID_3>0
) TX
ORDER BY Employee, Data

Facce sape'