Well subject to indexes and performance testing I do this
;with cte (ID) AS (
SELECT ID
FROM Test1 T1
WHERE @p_flag = 1
ANDT1.moveFlag = 26
UNION ALL
SELECT ID
FROM Test1 T1
WHERE @p_flag = 0
ANDT1.moveFlag <> 26)
SELECT ..
FROM cte T1
JOIN Test2 T2 ON T1.ID = T2.ID
Far away is close at hand in the images of elsewhere.
Anon.