sql - 在 SQL Server 中使用按大小写顺序在 select 语句中使用的函数时出错

Question

我有下面的表结构，我想通过使用 case 关键字按顺序使用我在 select 语句中使用的函数。但我收到错误消息，指出 FullName 是无效列。

CREATE TABLE STUDENTS
(
     ID INT IDENTITY(1, 1) ,
     FirstName VARCHAR(50) ,
     LastName VARCHAR(50)
)

向表 STUDENTS 中插入一些数据

INSERT  INTO STUDENTS
VALUES  ( 'kamal', 'sharma' )

INSERT  INTO STUDENTS
VALUES  ( 'john', 'jack' )

INSERT  INTO STUDENTS
VALUES  ( 'Rahul', 'Thomson' )

此语句有效

SELECT 
       ID
     , FirstName + ' ' + LastName AS FullName
FROM STUDENTS
ORDER BY FullName ASC

我需要这个，但它说 FullName 是无效列。

DECLARE @VAL INT  
SET @VAL = 1

SELECT ID
    ,FirstName + ' ' + LastName AS FullName
FROM STUDENTS
ORDER BY CASE 
        WHEN @VAL = 1
            THEN FullName
        END ASC
    ,CASE 
        WHEN @VAL = 2
            THEN FullName
        END DESC

Msg 207, Level 16, State 1, Line 10
Invalid column name 'FullName'.
Msg 207, Level 16, State 1, Line 14
Invalid column name 'FullName'.

预先感谢您的帮助。

Answer 1

在 SELECT 中定义的列别名不能在相应 ORDER BY 的表达式内部使用。

要么重复底层定义。

SELECT ID
    ,FirstName + ' ' + LastName AS FullName
FROM STUDENTS
ORDER BY CASE 
        WHEN @VAL = 1
            THEN FirstName + ' ' + LastName
        END ASC
    ,CASE 
        WHEN @VAL = 2
            THEN FirstName + ' ' + LastName
        END DESC

或者在不同级别定义别名。例如派生表或 CTE。

WITH T
     AS (SELECT ID,
                FirstName + ' ' + LastName AS FullName
         FROM   STUDENTS)
SELECT *
FROM   T
ORDER  BY CASE
            WHEN @VAL = 1 THEN FullName
          END ASC,
          CASE
            WHEN @VAL = 2 THEN FullName
          END DESC