SQL Query optimieren

[ms88]

SELECT m.1,m.2,m.3
FROM table1 m
LEFT JOIN table2 rc ON rc.mID = m.mID
LEFT JOIN table3 r ON rc.rID = r.rID
WHERE m.5 != '' AND
m.mID NOT
IN (
SELECT mID
FROM table2 WHERE mID IS NOT NULL
)

>   
  
Probier es mal mit not exists zb in die richtung:  
  
select m.1, m.2, m.3 from table1 m WHERE not exists (select \* from table2 WHERE mID=m.mID)