string strSQL = string.Empty;
if(DropDownList1.SelectedValue == "true")
    strSQL = "select * from tablename where checkstate=1";
else if(DropDownList1.SelectedValue == "false")
    strSQL = "select * from tablename where checkstate=0";
else if(DropDownList1.SelectedValue == "all")
    strSQL = "select * from tablename";