在 SELECT 列表中的其他位置引用别名

Question

我有以下疑问：

select FirstName, LastName,
    case
        when LastName = 'Jones'
        then 'N/A'
    end as Other,
    case 
        when Other is not null 
        then 1 
    else 0 as Flag

Flag 值取决于 Other，但由于 Other 是别名，因此 Flag 字段无法识别 Other。

我想，我可以在选择中使用选择。 Flag 有没有更好的方法来识别其他别名列？

Answer 1

由于查询的解析方式，您无法引用 SELECT 和 ORDER BY 之外的别名。典型的解决方法是将其埋入派生表中：

SELECT 
  FirstName, LastName, Other, 
  Flag = CASE WHEN Other IS NOT NULL THEN 1 ELSE 0 END
FROM 
(
  SELECT FirstName, LastName,
    CASE WHEN LastName = 'Jones'
     THEN 'N/A'
    END AS Other
  FROM dbo.table_name
) AS x;

Answer 2

您还可以使用公用表表达式 (CTE) 来完成此操作：

;WITH cte AS 
(
SELECT 
    firstname, 
    lastname, 
    CASE WHEN lastname = 'Jones' THEN 'N/A' 
    END AS Other 
FROM @t
) 
SELECT 
    firstname, 
    lastname, 
    CASE WHEN other IS NOT NULL THEN 1 
        ELSE 0 
    END AS Flag 
FROM cte

Answer 3

您必须在

SELECT

中使用

SELECT

：

SELECT FirstName, LastName, Other, 
    Case When Other is not null 
         then 1 
         else 0 END as Flag
FROM 
(
    select FirstName, LastName,
        Case
         When LastName = 'Jones'
         then 'N/A'
        End as Other
    from yourTable
) x

或者您可以在评估

CASE

字段时重复使用

Other

语句来表示

Flag

字段：

select FirstName, 
    LastName,
    Case 
        When LastName = 'Jones'
        then 'N/A'
    End as Other,
    case when LastName = 'Jones' -- your Other CASE code here
        then 1
        else 0
    END as flag 
from yourTable