Case statement in SQL Rownumber() Order By clause not working on varchar and int values

Question

Why doesn't this work?

DECLARE @temp table
(ShipNo int,
 Supplier varchar(10)
)

INSERT INTO @temp VALUES (1,'CFA')
INSERT INTO @temp VALUES (1, 'TFA')
INSERT INTO @temp VALUES (2, 'LRA')
INSERT INTO @temp VALUES (2, 'LRB')
INSERT INTO @temp VALUES (3, 'ABC')
INSERT INTO @temp VALUES (4, 'TFA')

Declare @OrderBy varchar(255)
sET @OrderBy = 'Supplier'



SELECT ROW_NUMBER() OVER (ORDER BY 
CASE @OrderBy
 WHEN 'Supplier' THEN Supplier
 WHEN 'ShipNo' THEN ShipNo
END
) AS RowNo,
ShipNo,
Supplier
FROM @temp

But if you cast the ShipNo to a varchar in the Case statement it DOES work?

Answer 1

From Books Online:

CASE
WHEN Boolean_expression THEN result_expression 
    [ ...n ] 
[ 
    ELSE else_result_expression 
] 
END

"The data types of else_result_expression and any result_expression must be the same or must be an implicit conversion."

So Supplier and ShipNo must be the same datatype.

Answer 2

I am aware that this is old post but this is for any one who tumbles upon this issue and is looking for a solution:

SELECT ROW_NUMBER() OVER (ORDER BY
CASE @OrderBy WHEN 'Supplier' THEN Supplier END CASE WHEN 'ShipNo' THEN ShipNo END )

basically you are putting each field in its own case. Do this only when their data type or value inside field differs for both the columns or when you are getting error such as

conversion failed when trying to convert int to varchar or varchar to int..