mysql variable

mysql variable

$query = "SELECT * ,
                  @amount:=(SELECT SUM((`ghi_qty`*`g_price`)) FROM `tbl_grn_has_item` ghi WHERE ghi.`idgrn`= grn.`idgrn`) AS amount,
                  (@amount-(@amount*(grn.g_discount/100))) AS `grandtotal`,
                  (@amount*(grn.g_discount/100)) AS `discount`
                  FROM `tbl_grn` grn 
                  INNER JOIN `tbl_delivery_note` dn ON grn.`iddelivery_note`= dn.`iddelivery_note` 
                  INNER JOIN `tbl_purchase_order` po ON dn.`idpurchase_order`= po.`idpurchase_order` 
                  INNER JOIN `tbl_user` tu ON po.`idposition`=tu.`iduser` WHERE grn.`g_status`='0' aND  po.`idposition`='{$user_id}'{$querypart}";