[firebird-support] Is such a CASE WHEN usage allowed?

[venussoftop]

Hi all

Can I use a CASE WHEN in the WHERE for a statement as below?  The idea was to 
either pull out records from the tSaleInvoice based on a list of PKs in a 
separate table.field (see my other IN .... post) if the value is not present 
then on the same records date and iBuyerID.

SELECT tSaleinvoice.*
   FROM tSaleInvoice
   WHERE CASE WHEN COALESCE (:cSaleInvIDs, '') = ''
      THEN iBuyerID = :iBuyerID AND CAST(tSaleInvoice.tDt AS DATE) = CAST(:tDt 
AS DATE)
      ELSE tSaleInvoice.iID IN (SELECT tSaleInvoiceCvrLtr.cSaleInvIDs
                                                  FROM tSaleInvoiceCvrLtr
                                                  WHERE tSaleInvoiceCvrLtr.iID 
= :vp_iID) END

So
SELECT tSaleInvoiceCvrLtr.*
   FROM tSaleInvoiceCvrLtr
   WHERE tSaleInvoiceCvrLtr.iID = 993
should allow me to bring out many records from tSaleInvoice on two criterias
1) on tSaleInvoiceCvrLtr.cSaleInvIDs is populated or if not
2) on tSaleInvoiceCvrLtr.iBuyerID and tSaleInvoiceCvrLtr.tDt

Hope I was able to explain my problem.  The first SELECT is from Report Manager 
thus the :fieldnames

Please advise

Thanks and regards
Bhavbhuti