如何结合两个SQL查询？

SELECt a.item_id, a.stock_balance, b.pcs_ordered, b.number_of_ordersFROM    (SELECt stock.item_id, Sum(stock.pcs) AS stock_balance     FROM stock     GROUP BY stock.item_id) aLEFT OUTER JOIN    (SELECt stock.item_id, Sum(stock.pcs) AS pcs_ordered,  Count(stock.item_id) AS number_of_orders     FROM stock    WHERe stock.operation = "order"    GROUP BY stock.item_id) bON a.item_id = b.item_id