Tsql 在SQL Server中转换MS Access动态列

我想将此MS Access动态查询示例转换为T-SQL：

SELECT 
    Name, 
    iif(expression='A', value, 0) as vA,
    iif(expression='B', value, 0) as vB,
    vA-vB as diff
FROM 
    MyTable

我尝试了这段代码，但编译器告诉我表达式

vA vB as diff

中的新动态列vA和vB无法识别

SELECT 
    Name,
    CASE WHEN expression = 'A' then value else 0 end AS vA,
    CASE WHEN expression = 'B' then value else 0 end AS vB,
    vA-vB as diff
FROM 
    MyTable

---

不能使用同一select语句中定义的别名。试试这个

SELECT NAME,
       CASE WHEN expression = 'A' THEN value ELSE 0 END               AS vA,
       CASE WHEN expression = 'B' THEN value ELSE 0 END               AS vB,
       ( CASE WHEN expression = 'A' THEN value ELSE 0 END ) - 
       ( CASE WHEN expression = 'B' THEN value ELSE 0 END ) AS diff
FROM   MyTable

或者作为

子选择进行查询

，并在

外部查询中查找
diff

SELECT vA,
       VB,
       vA - vB AS Diff
FROM   (SELECT NAME,
               CASE WHEN expression = 'A' THEN value ELSE 0 END AS vA,
               CASE WHEN expression = 'B' THEN value ELSE 0 END AS vB
        FROM   MyTable) A 

当行完成处理时，将定义别名。因此，它们在同一行的其他位置的“读取”上下文中不可用。在
SELECT
子句中可以使用别名，这是访问的许多特性之一，尽管据我所知，它们必须按顺序定义。这可能很有用，但如果查询中有一个与表字段同名的计算字段，这可能会是一场噩梦。