Question

所以这实际上不是我的代码，而只是我正在尝试做的一个例子。理想情况下，我可以使用INNER JOINS和外键关系来获取数据，但我不能在现实生活中 - 这只是一个简单的例子。

SELECT [EmployeeID],
       [DepartmentID],
       (SELECT Title FROM Depts WHERE ID = [DepartmentID]) AS Department, 
       (SELECT Name FROM DeptHeads WHERE DeptName = Department) AS DepartmentLead
FROM   Employees E

我从一个表（Employees）获取数据。

我在子查询的where子句中使用该表中的一列（DepartmentID），并从中创建别名（部门）

我正在尝试做与上面相同的事情，除了在where子句中使用该别名。

我收到错误说：

无效的列名称“部门”

有没有更好的方法让我这样做，或者解决这个问题？

Answer 1

您不能使用刚刚定义的别名。你可以：

SELECT * FROM (

    SELECT [EmployeeID],
               [DepartmentID],
               (SELECT Title FROM Depts WHERE ID = [DepartmentID]) AS Department, 
               (SELECT Name FROM DeptHeads WHERE DeptName = Department) AS DepartmentLead
    FROM   Employees E

) Base

WHERE Base.Department = ...

Answer 2

;WITH MyCTE AS
(
    SELECT [EmployeeID],
       [DepartmentID],
       (SELECT Title FROM Depts WHERE ID = [DepartmentID]) AS Department, 
       (SELECT Name FROM DeptHeads WHERE DeptName = Department) AS DepartmentLead
    FROM   Employees E
)
SELECT *
FROM   MyCTE
WHERE  Department = 'IT'

Answer 3

方法1：

SELECT [EmployeeID],
[DepartmentID],
[Department],
(SELECT Name FROM DeptHeads WHERE DeptName = Department) AS DepartmentLead
FROM
(SELECT [EmployeeID],
       [DepartmentID],
       (SELECT Title FROM Depts WHERE ID = [DepartmentID]) AS Department, 
       FROM   Employees E ) E2

在where子句

中使用[Department]别名

SELECT [EmployeeID],
[DepartmentID],
[Department],
(SELECT Name FROM DeptHeads WHERE DeptName = Department) AS DepartmentLead
FROM
(SELECT [EmployeeID],
       [DepartmentID],
       (SELECT Title FROM Depts WHERE ID = [DepartmentID]) AS Department, 
       FROM   Employees E ) E2
WHERE E2.Department = 'XYZ'

方法2：

SELECT E.[EmployeeID],
       E.[DepartmentID],
       D.Title AS Department, 
       DH.Name AS DepartmentLead
FROM   Employees E
LEFT JOIN Depts D ON E.[DepartmentID] = D.ID
LEFT JOIN DeptHeads DH ON D.Title = DH.DeptName

SQL - 在子查询的where子句中使用别名

3 个答案: