Try this

;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);

Take from