sql server - About 'CASE' a conversion failure

Question

While using 'CASE' I encountered with problem that I'D never heard.
The problem is so: I'M using 'CASE' statement in 'SELECT' clause as a field. When calling columns with int data type, it's successful; However, When calling a varchar or datetime, it's unsuccessful.
?n the error result, it's said me to have marked the temporary field, including 'CASE' statement, as int, But not so. Please help...

The 'STORED PROCEDURE' Content

CREATE PROC spRegistry_PersonsGetField
@ID int,
@Field varchar(100)
AS
SELECT CASE @Field 
WHEN 'Name' THEN Name
WHEN 'Surname' THEN Surname
WHEN 'Username' THEN Username
WHEN 'Password' THEN Password
WHEN 'Status' THEN dbo.fnStatus(Status)
WHEN 'EMail' THEN EMail
WHEN 'Tel' THEN dbo.fnIsEmpty(Tel)
WHEN 'Address' THEN dbo.fnIsEmpty(Address)
WHEN 'City' THEN dbo.fnIsEmpty(City)
WHEN 'BirthDate' THEN dbo.fnDate(BirthDate)
WHEN 'Gender' THEN dbo.fnGender(Gender)
WHEN 'Lang' THEN Lang
ELSE NULL
END
AS [Alan] varchar(200)
FROM Registry_Persons
WHERE ID = @ID

My Query

EXEC spRegistry_PersonsGetField 1, 'BirthDate'

The Error Message

Conversion failed when converting the varchar value '04/11/2011' to data type int.

Answer 1

T-SQL's Precedence rules apply when a CASE is evaluated in order to work out what the single returned datatype is to be. Since int has a higher precedence than varchar an implicit conversion to int is attempted & subsequently fails.

To fix this, convert your int or datetime expressions to varchar.