为什么没有嵌套CASE语句不起作用？ [重复]
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这个问题在这里已有答案：
我需要根据列选择动态地对结果进行排序。我知道下面只有第一个查询有效，第二个查询失败。但是我无法理解它失败的原因，根据我对@SortDirection值的理解，它应该给ASC或DESC，它将附加到返回列名的早期CASE。你能解释一下它失败的原因吗？
create table EmpData
(
EmpID int identity,
FName varchar(20),
LName varchar(20),
Email varchar(30),
City varchar(20),
DOB date
)
GO
INSERT INTO EmpData values ('Mark','Smith','Mark.s@email.com','London','02-Jun-1980')
INSERT INTO EmpData values ('Mark','Marsh','Mark.m@email.com','Paris','02-Jul-1981')
INSERT INTO EmpData values ('Steve','Elgar','Steve.e@email.com','Canada','12-Jun-1980')
INSERT INTO EmpData values ('Michael','Jones','Michael.j@email.com','France','22-Jan-1980')
GO
----------QUERY 1--------
DECLARE @SortByColumn varchar(20)='City',
@SortDirection varchar(5)='ASC'
SELECT EmpID,FName AS FirstName, LName AS LastName, Email, City, DOB as 'Date-Of-Birth'
FROM EmpData
ORDER BY
CASE @SortDirection WHEN 'ASC' THEN
CASE @SortByColumn
WHEN 'FirstName' THEN FName
WHEN 'LastName' THEN LName
WHEN 'Email' THEN Email
WHEN 'City' THEN City
ELSE ''
END
END,
CASE @SortDirection WHEN 'DESC' THEN
CASE @SortByColumn
WHEN 'FirstName' THEN FName
WHEN 'LastName' THEN LName
WHEN 'Email' THEN Email
WHEN 'City' THEN City
ELSE ''
END
END DESC
GO
----------QUERY 2--------
DECLARE @SortByColumn varchar(20)='City',
@SortDirection varchar(5)='ASC'
SELECT EmpID,FName AS FirstName, LName AS LastName, Email, City, DOB as 'Date-Of-Birth'
FROM EmpData
ORDER BY
CASE @SortByColumn
WHEN 'FirstName' THEN FName
WHEN 'LastName' THEN LName
WHEN 'Email' THEN Email
WHEN 'City' THEN City
ELSE ''
END
CASE @SortDirection
WHEN 'DESC' THEN DESC
ELSE ASC
END
答案
试试这样吧
DECLARE @SortByColumn varchar(20)='City',
@SortDirection varchar(5)='asc'
;WITH CTE
AS
(
SELECT
Seq1 = ROW_NUMBER() OVER(ORDER BY
CASE @SortByColumn
WHEN 'FirstName' THEN FName
WHEN 'LastName' THEN LName
WHEN 'Email' THEN Email
WHEN 'City' THEN City
ELSE ''
END ASC),
Seq2 = ROW_NUMBER() OVER(ORDER BY
CASE @SortByColumn
WHEN 'FirstName' THEN FName
WHEN 'LastName' THEN LName
WHEN 'Email' THEN Email
WHEN 'City' THEN City
ELSE ''
END DESC),
EmpID,FName AS FirstName, LName AS LastName, Email, City, DOB as 'Date-Of-Birth'
FROM EmpData
)
SELECT
*
FROM CTE
ORDER BY CASE @SortDirection WHEN 'DESC' THEN Seq2 ELSE Seq1 END
另一答案
在这种情况下最好使用动态SQL
Declare @sql nvarchar(max)
declare @orderby nvarchar(100) = ' ORDER BY '
DECLARE @SortByColumn varchar(20) = 'City'
declare @SortDirection varchar(5) = 'ASC'
set @sql = '
SELECT EmpID,FName AS FirstName, LName AS LastName, Email, City, DOB as ''Date-Of-Birth''
FROM EmpData
' +
CASE @SortByColumn
WHEN 'FirstName' THEN @orderby + ' FName ' + @SortDirection
WHEN 'LastName' THEN @orderby + ' LName ' + @SortDirection
WHEN 'Email' THEN @orderby + ' Email ' + @SortDirection
WHEN 'City' THEN @orderby + ' City ' + @SortDirection
ELSE ''
END
exec sp_executesql @sql
由于ASC或DESC是SQL子句，因此我们不能在CASE语句中使用它们
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