Mysql 得到一个错误，说有'；如果字段列表中有未知表，则该表将存在

我试图通过查找差异来输出给客户的折扣金额，但当我当前数据库中有一个名为

item

的表时，我在字段列表中不断得到

Error\1109未知表'item'

我正在使用此查询：

SELECT orderpay.orderid , cust.name , (orderpay.totalpay - actualCost) as `discount amount`
FROM cust , orderpay , tran , item, 
  (SELECT sum(item.unitprice * tran.quantity) ) as actualCost
WHERE item.itemid = tran.itemid AND (orderpay.totalpay - actualCost) > 0
GROUP BY item.itemid

以下是我的表格结构：

cust
+---------+-------------+------+-----+---------+-------+
|字段|类型|空|键|默认|额外|
+---------+-------------+------+-----+---------+-------+
|custid国际（5）编号PRI 0|
|名称| varchar（22）|是| |空||
|电子邮件| varchar（33）|是| |空||
|地址| varchar（29）|是| |空||
|城市|瓦尔查尔（21）|是| |空||
|国家|瓦尔查尔（14）|是| |空||
+---------+-------------+------+-----+---------+-------+
项目
+-----------+--------------+------+-----+---------+-------+
|字段|类型|空|键|默认|额外|
+-----------+--------------+------+-----+---------+-------+
|项目ID | int（5）| NO | PRI | 0 ||
|名称| varchar（94）|是| |空||
|单价|十进制（6,2）|是| |空||
|cat | varchar（12）| YES | | NULL ||
+-----------+--------------+------+-----+---------+-------+
订单工资
+----------+---------------+------+-----+---------+-------+
|字段|类型|空|键|默认|额外|
+----------+---------------+------+-----+---------+-------+
|医嘱ID | int（5）| NO | PRI | 0 ||
|日期| varchar（10）|是| |空||
|custid | int（5）| YES | | NULL ||
|cardnum | varchar（25）| YES | | NULL ||
|卡片类型| varchar（25）|是| |空||
|工资总额|十进制（10,2）|是| |空||
+----------+---------------+------+-----+---------+-------+
特兰
+----------+--------+------+-----+---------+-------+
|字段|类型|空|键|默认|额外|
+----------+--------+------+-----+---------+-------+
|医嘱ID | int（5）| NO | PRI | 0 ||
|项目ID | int（5）| NO | PRI | 0 ||
|数量|整数（2）|是| |空||
+----------+--------+------+-----+---------+-------+
考虑如下更改查询

SELECT *, 
(totalpay - computed_price) as `discount amount`
FROM 
(
SELECT orderpay.orderid , 
cust.name ,
orderpay.totalpay , 
sum(item.unitprice * `tran`.quantity) as computed_price     
FROM cust JOIN `tran` ON item.itemid = tran.itemid
JOIN orderpay ON cust.some_column = orderpay.some_column      
JOIN item ON  cust.some_column = item.some_column      
GROUP BY item.itemid
) tab
WHERE (totalpay - computed_price) > 0

作为旁注，你不应该用它来计算总价格，因为如果价格改变了（而且它们确实在时间上发生了变化），你的计算就会一团糟。相反，在创建订单时，您应该记住每个订单的折扣

非常重要

还有一点让我害怕的是，你有一个名为cardnum | varchar（25）的字段，你永远不应该保留抄送号码。
我认为没有子查询你就做不到。不是那种子查询：），我会试试看。是否有多个订单，你想找到所有订单的总数，或者您正在尝试获取每个订单的折扣？尝试获取具有任何类型折扣的所有订单的客户名称、ID和折扣金额选择总和（item.unitprice*tran.quantity）），这在清除信用卡号存储中不是一个很好的查询点！
Select totalpaid.custid, (totalprices.total - totalpaid.total) as `discount` FROM 
(
     SELECT sum(totalpay) as total, custid 
     FROM orderpay 
     GROUP BY custid
) as totalpaid JOIN 
(
     SELECT sum(item.unitprice * tran.quantity) as total, orderpay.custid 
     FROM orderpay JOIN tran ON orderpay.orderid=tran.orderid JOIN item ON tran.itemid item.itemid
     GROUP BY orderpay.custid
) as totalprices ON totalpaid.custid = orderpay.custid