Sounds like you need a couple of left joins and a case - this should do...

SELECT

s.sno

,CASEWHEN e.sno IS NOT NULL THEN

'Y'

WHEN n.sno IS NOT NULL THEN

'N'

WHEN e.sno IS NULL AND n.sno IS NULL THEN

'N/A'

END

FROM student s

LEFT JOIN StudentDrillEnrolled e

ON s.sno=e.sno

LEFT JOIN StudentDrillNotEnrolled n

ON s.sno=n.sno;