Or you can use EXISTS:
SELECT
sno, sname, address,
CASE
WHEN EXISTS(SELECT 1 FROM dbo.StudentDrillEnrolled sde WHERE sde.sno = s.sno) THEN 'Y'
WHEN EXISTS(SELECT 1 FROM dbo.StudentDrillNotEnrolled sdne WHERE sdne.sno = s.sno) THEN 'N'
ELSE 'N/A'
END AS Enrolled
FROM dbo.student s
SQL DBA,SQL Server MVP(07, 08, 09) "It's a dog-eat-dog world, and I'm wearing Milk-Bone underwear." "Norm", on "Cheers". Also from "Cheers", from "Carla": "You need to know 3 things about Tortelli men: Tortelli men draw women like flies; Tortelli men treat women like flies; Tortelli men's brains are in their flies".