sql - SQL - 指定字段可能与多个表相关

Question

我是 SQL 的初学者，并且一直在玩它。有两张桌子。一呼Customers一呼Orders。我正在计算有多少订单与客户 ID 相关，但它不断抛出错误。我有以下代码：

SELECT CustomerID, COUNT (orders.OrderID) AS TotalOrders
FROM Customers AS cust, Orders AS orders
WHERE cust.CustomerID = orders.CustomerID

它会引发一个错误，即该CustomerID字段可能出现在多个表中。

例如，如果我将其更改SELECT CustomerID为ContactName，则查询将正常工作。但是，我知道问题在于该CustomerID字段同时在表Customers和Orders表中。提前感谢您的帮助。

Answer 1

试试这个

SELECT cust.CustomerID, COUNT (orders.OrderID) AS TotalOrders
FROM Customers AS cust
JOIN Orders AS orders ON cust.CustomerID = orders.CustomerID
GROUP BY cust.CustomerID

此查询使用ANSI 标准的显式 JOIN语法。这是最佳实践。您唯一缺少的是GROUP BY对聚合进行分组。您还需要确保将表名添加到列的前缀以防止歧义。

Answer 2

您必须使用表名或别名来限定字段名称...建议使用别名。此外，建议您切换到 ANSI-92 Join 语法。

  SELECT c.CustomerID, COUNT (o.OrderID) AS TotalOrders
  FROM Customers c Join Orders o 
     On c.CustomerID = o.CustomerID
  GROUP BY c.CustomerID

Answer 3

应该：

SELECT cust.CustomerID, COUNT (orders.OrderID) AS TotalOrders
FROM Customers AS cust, Orders AS orders
WHERE cust.CustomerID = orders.CustomerID