sql-server-2005 - 使用单个 SELECT 语句和 CASE 将表值分配给多个变量？

Question

我试图通过使用具有多个 CASE 语句的单个 SELECT 将查找表中包含的值分配给多个变量。

该表是一个具有两列的查找表，如下所示：

[GreekAlphabetastic]

  SystemID    Descriptor
  --------    ----------
  1           Alpha
  2           Beta
  3           Epsilon

这是我的语法：

SELECT 

    @VariableTheFirst = 
        CASE
            WHEN myField = 'Alpha' THEN tbl.SystemID
        END,

    @VariableTheSecond = 
        CASE
            WHEN myField = 'Beta' THEN tbl.SystemID
        END,

    @VariableTheThird = 
        CASE
            WHEN myField = 'Epsilon' THEN tbl.SystemID
        END

FROM GreekAlphabetastic tbl

但是，当我在该语句执行后检查变量时，我希望每个变量都被分配适当的值，但只有最后一个被分配了值。

SELECT 
    @VariableTheFirst AS First, 
    @VariableTheSecond AS Second, 
    @VariableTheThird AS Third

Results:

    First    Second    Third
    NULL     NULL      3

我究竟做错了什么？

Answer 1

从 SELECT 对局部变量进行赋值时，只有最后处理的行会影响变量。对于第三行，CASE myField = 'Alpha'andCASE myField = 'Beta'为假，变量设置为 NULL。为CASE myField = 'Epsilon'真，@VariableTheThird 被赋值为 3。

如果你想让这个工作这样做：

SELECT @VariableTheFirst = tbl.SystemID WHERE myField = 'Alpha'
SELECT @VariableTheSecond = tbl.SystemID WHERE myField = 'Beta'
SELECT @VariableTheThird = tbl.SystemID WHERE myField = 'Epsilon'

Answer 2

前 2 个变量在分配后被重置为 null。即，当它达到 Epsilon 记录时，前 2 个变量被分配为 null，因为在 CASE 语句中没有什么可以阻止它。

所以，试试这个：

SELECT     
    @VariableTheFirst = 
        CASE
            WHEN Descriptor = 'Alpha' THEN tbl.SystemID
            ELSE @VariableTheFirst
        END,

    @VariableTheSecond = 
        CASE
            WHEN Descriptor = 'Beta' THEN tbl.SystemID
            ELSE @VariableTheSecond
        END,

    @VariableTheThird = 
        CASE
            WHEN Descriptor = 'Epsilon' THEN tbl.SystemID
            ELSE @VariableTheThird
        END
FROM GreekAlphabetastic tbl

Answer 3

SELECT     
        CASE
            WHEN Descriptor = 'Alpha' THEN @VariableTheFirst = isnull(@VariableTheFirst,tbl.SystemID)
            WHEN Descriptor = 'Beta' THEN @VariableTheSecond = isnull(@VariableTheSecond,tbl.SystemID)
            WHEN Descriptor = 'Epsilon' THEN @VariableTheThird = isnull(@VariableTheThird,tbl.SystemID)
        END
FROM GreekAlphabetastic tbl