Original:
SELECT MAX(lastdatatimestamp) as lastdatatimestamp FROM (
SELECT MAX(a.timestamp) AS lastdatatimestamp FROM 103_alert a
UNION SELECT MAX(l.timestamp) AS lastdatatimestamp FROM 103_linear l WHERE l.data_type IN ( 1, 2 )
UNION SELECT MAX(p.timestamp) AS lastdatatimestamp FROM 103_photovoltaics p WHERE p.data_type IN ( 1, 2 )
) AS lastdatatimestamp


Converted (not working)
SELECT MAX(lastdatatimestamp) as lastdatatimestamp FROM (
SELECT a.timestamp AS lastdatatimestamp FROM 103_alert a ORDER BY a.timestamp desc LIMIT 1
UNION SELECT l.timestamp AS lastdatatimestamp FROM 103_linear l WHERE l.data_type IN ( 1, 2 ) ORDER BY l.timestamp desc LIMIT 1
UNION SELECT p.timestamp AS lastdatatimestamp FROM 103_photovoltaics p WHERE p.data_type IN ( 1, 2 )  ORDER BY p.timestamp desc LIMIT 1
) AS lastdatatimestamp