sql语句查询是有关时间的运算

SELECT WmsStock.sku_id,WmsStock.stock_num,WmsStock.product_date,WmsStock.expire_date,WmsStock.cost_price,GoodsSku.sku_name,GoodsSku.barcode, CONVERT(((UNIX_TIMESTAMP(WmsStock.expire_date)-UNIX_TIMESTAMP(WmsStock.product_date))/(60*60*24)),SIGNED) AS 保质期, WmsStock.stock_num*WmsStock.cost_price AS 总价格 FROM wms_stock AS WmsStock LEFT JOIN goods_sku AS GoodsSku ON WmsStock.sku_id=GoodsSku.sku_idSELECT WmsStock.sku_id,WmsStock.stock_num,WmsSt