Question

我有以下查询：

    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，Flag字段无法识别Other。

我想，我可以在选择中使用选择。 Flag有更好的方法来识别Other alias列吗？

Answer 1

由于解析查询的方式，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在派生表中：

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

您也可以使用公用表表达式（CTE）执行此操作：

;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字段{：1}}字段：

Flag

在SELECT列表中的其他位置引用别名

3 个答案: