Hi,

 

i suppose this one is much faster

 

SELECT P.ID

, CASE WHEN EXISTS(SELECT * FROM  CHILD C WHERE C.ID_PARENT=P.ID AND C.MYFIELD=3)

 

) 

THEN 1 ELSE 0 END AS ISDATA

  FROM PARENT P

...........

INNER JOIN TABLE3 T3 ON P.ID=T3.....

INNER JOIN TABLE3 T4 ON P.ID=T4.....