This should help

WHERE a.connected = 0 AND ( dt.connected IS NOT NULL OR b.id IS NULL )

Even the below code will solve your requirement if you are using SQL Server 2005+

SELECT*

FROMforums AS a

LEFT OUTER JOIN(

SELECTROW_NUMBER() OVER ( PARTITION BY connected ORDER BY datum_updated DESC ) AS RN, *

FROMforums

) b ON a.id = b.connected AND b.RN = 1

WHEREa.connected = 0

Edit: Added alternate code