how to retrive boolean column based on drop down list value

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SQL-Server-2008R2

i have a form in which i want to show the data based on user's selection..in this form there is a drop down list with items

'posted','drafted','all'. when user select posted i want to retrieve data by condition Voucher status=true, and when user select drafted i want to retrieve data by condition Voucher status=false. and when user select all i want to retrieve data by condition Voucher status=true or false or null.ie in third display all.(dont want to consider voucher status)

Pls help me to solve this query..

my complete query is here..

SELECT  VoucherTypes.types, VoucherHead.VoucherNo, VoucherHead.VoucherDate, SUM(VoucherDetail.Debit) AS debit, SUM(VoucherDetail.Credit) AS credit, VoucherHead.VoucherStatus
FROM  VoucherHead INNER JOIN VoucherTypes ON VoucherHead.VoucherType = VoucherTypes.vtypeid INNER JOIN
VoucherDetail ON VoucherHead.VoucherID = VoucherDetail.VoucherID
WHERE (VoucherHead.VoucherDate >= @fromdate) AND (VoucherHead.VoucherDate <= @todate) AND (VoucherTypes.types = @vtype or @vtype='0') AND (VoucherHead.VoucherStatus = @st) AND (VoucherHead.Branchno = @branchid) 
GROUP BY VoucherTypes.types, VoucherHead.VoucherNo, VoucherHead.VoucherDate, VoucherHead.VoucherStatus

here

if (status == 1)
    st = true;

if (status == 2)
   st = false;

where status=drp.selectedindex;

but this works only selecting 'posted' or 'drafted'.. it also be worked when selecting 'all'...

is this can de done using 'case'..i dont know how to use it...pls help me...