--== SAMPLE DATA
SELECT StudID, RollNo
INTO #tableA
FROM (VALUES('Alpha','D1234'),('beta','A1122'),('charlie','D1234'),('bravo','C1342'),
            ('tom','B1964'),('harry','A1122'))a(StudID, RollNo);

--== SAMPLE DATA
SELECT RollNo, SubjectCode
INTO #tableB
FROM (VALUES('D1234',1001),('A1122',4001),('D1234',2001),('C1342',5001),('B1964',6001),
            ('A1122',3001))a(RollNo, SubjectCode);

--== ACTUAL SOLUTION
SELECT tblA.StudID, tblA.RollNo, subQuery.SubjectCode
FROM #tableA tblA
CROSS APPLY (SELECT TOP 1 RollNo, SubjectCode
             FROM #tableB tblB
             WHERE tblA.RollNo = tblB.RollNo) subQuery;

--========= Student table

IF OBJECT_ID('tempdb..#Student') IS NOT NULL
BEGIN
	DROP TABLE #Student
END
GO
CREATE TABLE #Student
(
StudID VARCHAR(100),
RollNo VARCHAR(100)
)
GO
INSERT INTO #Student VALUES('Alpha', 'D1234')
INSERT INTO #Student VALUES('beta', 'A1122')
INSERT INTO #Student VALUES('charlie', 'D1234')
INSERT INTO #Student VALUES('bravo', 'C1342')
INSERT INTO #Student VALUES('tom', 'B1964')
INSERT INTO #Student VALUES('harry', 'A1122')
Go

--========= StudentSubjects table
IF OBJECT_ID('tempdb..#StudentSubjects') IS NOT NULL
BEGIN
	DROP TABLE #StudentSubjects
END
GO
CREATE TABLE #StudentSubjects
(
RollNo VARCHAR(100),
SubjectCode VARCHAR(100),
)
GO 
INSERT INTO #StudentSubjects VALUES ('D1234', 1001)
INSERT INTO #StudentSubjects VALUES ('A1122', 4001)
INSERT INTO #StudentSubjects VALUES ('D1234', 2001)
INSERT INTO #StudentSubjects VALUES ('C1342', 5001)
INSERT INTO #StudentSubjects VALUES ('B1964', 6001)
INSERT INTO #StudentSubjects VALUES ('A1122', 3001)
Go

--======= Solution:
;WITH MyCTE (RollNo, LowestSubjectCode)
AS
	(
		SELECT RollNo, MIN (SubjectCode) AS LowestSubjectCode FROM #StudentSubjects GROUP BY RollNo
	)
SELECT	S.StudId,S.RollNo,MC.LowestSubjectCode
FROM	#Student S INNER JOIN MyCTE MC
ON		S.RollNo = MC.RollNo