如何在联合查询中使用别名

Question

下面的查询没有问题

SELECT [TblA].fieldA, 
iif(isnull([fieldA]),"",IIF(Len([fieldA]) - Len(Replace([fieldA], ",", ""))<1,[fieldA],Left([fieldA],InStr([fieldA],",")-1))) AS a, 
iif(isnull([fieldA]),"",IIF(Len([fieldA]) - Len(Replace([fieldA], ",", ""))<1,'',IIF(Len([fieldA]) - Len(Replace([fieldA], ",", ""))=1,Right([fieldA],len([fieldA])-len([a])-1),           Mid([fieldA],Len([a])+2,Instr(Len([a])+2,[fieldA],",")-Len([a])-2)))) AS b, 
iif(isnull([fieldA]),"",IIF(Len([fieldA]) - Len(Replace([fieldA], ",", ""))<2,'',IIF(Len([fieldA]) - Len(Replace([fieldA], ",", ""))=2,Right([fieldA],len([fieldA])-len([a])-len([b])-2),  Mid([fieldA],Len([a])+Len([b])+3,Instr(Len([a])+Len([b])+3,[fieldA],",")-Len([a])-Len([b])-3)))) AS c, 
iif(isnull([fieldA]),"",IIF(Len([fieldA]) - Len(Replace([fieldA], ",", ""))<3,'',IIF(Len([fieldA]) - Len(Replace([fieldA], ",", ""))=3,Right([fieldA],len([fieldA])-len([a])-len([b])-len([c])-3),  ""))) AS d
FROM TblA;

但是，当我尝试在联合查询中执行相同操作时，它不起作用（Enter Parameter Value for a）

SELECT iif(isnull([fldA]),"",IIF(Len([fldA]) - Len(Replace([fldA], ",", ""))<1,[fldA],Left([fldA],InStr([fldA],",")-1))) AS a FROM tblA
UNION
SELECT iif(isnull([fldA]),"",IIF(Len([fldA]) - Len(Replace([fldA], ",", ""))<1,'',IIF(Len([fldA]) - Len(Replace([fldA], ",", ""))=1,Right([fldA],len([fldA])-len([a])-1),Mid([fldA],Len([a])+2,Instr(Len([a])+2,[fldA],",")-Len([a])-2)))) AS b FROM tblA
UNION
SELECT iif(isnull([fldA]),"",IIF(Len([fldA]) - Len(Replace([fldA], ",", ""))<2,'',IIF(Len([fldA]) - Len(Replace([fldA], ",", ""))=2,Right([fldA],len([fldA])-len([a])-len([b])-2),Mid([fldA],Len([a])+Len([b])+3,Instr(Len([a])+Len([b])+3,[fldA],",")-Len([a])-Len([b])-3)))) AS c FROM tblA
UNION
SELECT iif(isnull([fldA]),"",IIF(Len([fldA]) - Len(Replace([fldA], ",", ""))<3,'',IIF(Len([fldA]) - Len(Replace([fldA], ",", ""))=3,Right([fldA],len([fldA])-len([a])-len([b])-len([c])-3),  ""))) As d FROM tblA

我的问题是：
有没有办法在 Union 查询中使用别名（例如 As a ）

Answer 1

无法从一个 SELECT 引用另一个 SELECT 中的字段（别名与否）。这些不是嵌套语句。此外，只有第一个 SELECT 定义字段名称。后续的 AS 子句将被忽略，可以删除。

在进行这些计算的地方构建第一个查询对象。然后使用第一个查询作为源构建 UNION。

SELECT a AS Data, "A" AS Category FROM query
UNION SELECT b, "B" FROM query
UNION SELECT c, "C" FROM query
UNION SELECT d, "D" FROM query;

UNION 不允许重复行。使用 UNION ALL 包括所有记录，即使有重复项。

如何在联合查询中使用别名

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