Why CASE statement doesn't work without nesting? [duplicate] - sql-server-2008

Question

This question already has answers here:
Dynamic Sorting within SQL Stored Procedures
(15 answers)
Closed 5 years ago.
I have a requirement to sort the results dynamically based on the column selection. I know that only first query below works and 2nd one fails. But am unable to understand why it fails, as per my understanding based on #SortDirection value it should give ASC or DESC, which will get appended to the earlier CASE which is returning column name. Can you please explain why it fails?
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

Answer 1

Try like this
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
DEMO

Answer 2

It is better to use dynamic SQL in such cases
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
Since ASC or DESC is SQL clauses, we can not use them in CASE statements