sql filtering results of alias column [duplicate]

Question

How can we filter for an Alias name? For example, in query below, for alias OLB, if we want to say WHERE OLB > 0

SELECT
loan.lnr
,loan.lamount - SUM (memrepay.mprinc) AS OLB
,lstatus

FROM
 loan
  LEFT OUTER JOIN memrepay
  ON loan.lnr = memrepay.lnr

WHERE
loan.tstart >= N'2013-07-07T14:00:00' 

GROUP BY loan.lnr, lamount, lstatus

Order By lnr

Answer 1

Use Derived table

select t.* from
(
SELECT
loan.lnr
,loan.lamount - SUM (memrepay.mprinc) AS OLB
,lstatus

FROM
 loan
  LEFT OUTER JOIN memrepay
  ON loan.lnr = memrepay.lnr

WHERE
loan.tstart >= N'2013-07-07T14:00:00' 

GROUP BY loan.lnr, lamount, lstatus
) as t where OLB>0
Order By lnr

or use HAVING clause and use the expression

SELECT
loan.lnr
,loan.lamount - SUM (memrepay.mprinc) AS OLB
,lstatus

FROM
 loan
  LEFT OUTER JOIN memrepay
  ON loan.lnr = memrepay.lnr
WHERE
loan.tstart >= N'2013-07-07T14:00:00' 

GROUP BY loan.lnr, lamount, lstatus
HAVING loan.lamount - SUM (memrepay.mprinc) >0
Order By lnr