Question

我在W3Schools的this示例游乐场页面上尝试了以下SQL。

SELECT CustomerID AS Id, CustomerName AS Customer
FROM Customers
WHERE Customer="Alfreds Futterkiste";

但我得到No value given for one or more required parameters.作为回应。如果我使用CustomerName而不是别名，它就可以工作。

这是游乐场测试页面的失败还是不可能？

Answer 1

查看您的代码：

SELECT CustomerID AS Id, CustomerName AS Customer
FROM Customers
WHERE Customer="Alfreds Futterkiste";

要为客户表添加别名，您可以执行以下操作：

SELECT c.CustomerID AS Id, c.CustomerName AS Customer
    FROM Customers c
    WHERE c.CustomerName="Alfreds Futterkiste";

其中c现在是Customers

的别名

不能在where子句中使用列别名，因为where子句在select之前执行。

Answer 2

很丑，但在更大，更复杂的查询中可以很方便

SELECT * FROM
(SELECT CustomerID AS ID, CustomerName AS Customer
FROM Customers) as A

WHERE Customer = "Alfreds Futterkiste";

Answer 3

在select之前评估WHERE子句。因此，where子句不知道您使用的别名。

因此您需要使用原始列名：

SELECT CustomerID AS Id, CustomerName AS Customer
FROM Customers
WHERE CustomerName="Alfreds Futterkiste";

如果必须在where子句中使用别名，则可以使用子查询或CTE（过度杀伤并可能导致查询速度变慢）：

SELECT * from (
    SELECT CustomerID AS Id, CustomerName AS Customer
    FROM Customers
) t WHERE Customer = "Alfreds Futterkiste";

Answer 4

否您无法在WHERE子句中引用列别名。