在SQL Server 2016中使用STUFF函数时将INT转换为字符串

Question

I'm trying to convert an INT but it is having an issue with the conversion.

Conversion failed when converting the nvarchar value '245428,246425' to data type int .

The query I am using:

SELECT STUFF
       ( 
        (
            SELECT DISTINCT ',' + CONVERT(VARCHAR(20), NumField) 
            FROM Table A 
            WHERE ID = 218554 
            FOR XML PATH('')
        ) ,1,1,''
       )

I use this as a subquery in a larger table like so:

SELECT 
    Field1,
    Field2, 
    CASE WHEN criteria = '1'
         THEN     (SELECT STUFF( 
            (
                SELECT DISTINCT ',' + CONVERT(VARCHAR(20), NumField) 
                FROM Table A 
                WHERE ID = 218554 
                FOR XML PATH('')
            ) ,1,1,''
           ))
    END 
FROM 
    Table B

The STUFF query runs fine when it's executed on it's own but when I run it in the full query it comes up with the conversion error.

Answer 1

I don't think you are not showing the full query -- or at least the full case expression. A case expression returns a single value with a single type.

When there are type conflicts, then SQL Server has to determine the single overall type, according to its rules. If one then returns an integer and another returns a string, then the case expression is an integer ( not a string). So, the string is converted to an integer.

You can see this problem with much simpler logic:

select (case when 1=1 then 'a' else 0 end)

Even though the else is never execution, the type of the expression is determined at compile time -- and 'a' cannot be converted to an integer.