You could change from WHERE custID = to custID IN
select salesDetails.bill_no,custDetails.custName,custDetails.custAdd from salesDetails,custDetails where custDetails.custID IN (select salesDetails.custID from salesDetails where slsDate between #" + DTP1.Value.Date.ToShortDateString() + "# and #" +DTP2.Value.Date.ToShortDateString() + "#)