How To Recall Column Name Defined In CASE Function

Hello all,

I need to refine a query in which one of the search conditions would depend on the value evaluated from the CASE function in SELECT statement. This returned column is named as "SLA". Now, the problem is I don't know how to recall this column in the WHERE clause as to do refinement. When I code it like SLA = @Term, SQL Server returned an error: Invalid column name 'SLA'

If anyone knows a solution, please kindly let me know.

Thank you!

Here is the sample code:

ALTER PROCEDURE [dbo].[sp_premium_register]
@PolicyType AS VARCHAR(10),
@ReportFrom AS DATETIME,
@ReportTo AS DATETIME,
@Business AS VARCHAR(1),
@Term AS VARCHAR(1)

SELECT
ColumnA,
ColumnB,

SLA =
CASE
WHEN DateDiff(day, P.EffectiveFrom, P.EffectiveTo) > 1 THEN 'L'
WHEN DateDiff(day, P.EffectiveFrom, P.EffectiveTo) <= 1 THEN 'S'
END

FROM DNIssue D
LEFT OUTER JOIN KILRIShare S
ON (D.PolicyNo = S.PolicyNo AND D.PolicyType = S.PolicyType AND D.Origin = S.Origin AND D.EndorsementNo = S.EndrNo AND D.PartyNo = S.RINo)
LEFT OUTER JOIN KILPolicy P
ON (D.PolicyNo = P.PolicyNo AND D.PolicyType = P.PolicyType AND D.Origin = P.Origin AND D.EndorsementNo = P.EndrNo)
LEFT OUTER JOIN v_report_KILDNFund F
ON (D.DebitNote = F.DebitNote)
LEFT OUTER JOIN PolicyProfile R
ON R.Origin = D.Origin AND R.PolicyType = D.PolicyType


WHERE
SLA = @Term

Order by D.PolicyType, D.DebitNote, D.Origin, D.PolicyNo, D.EndorsementNo, D.EntryDate