;WITH Missing (missnum, maxid)
AS
(
 SELECT 
	1 AS missnum, (select max(RegisterNo ) 
 FROM 
	 dbo.studentMaster)
 UNION ALL
 SELECT 
	missnum + 1, maxid 
 FROM 
	Missing
 WHERE 
	missnum < maxid
)
SELECT missnum
FROM 
	Missing
LEFT OUTER JOIN 
	dbo.studentMaster tt on tt.RegisterNo = Missing.missnum
WHERE 
	tt.RegisterNo is NULL
OPTION (MAXRECURSION 0);