如何在存储过程和MSSQL中的函数中将列名作为参数发送

Question

I'm trying to create a stored procedure in mssql (sql server) which takes the params of table name and column name. The stored proc should update given table and column name with base 64 value. I was able to create the function to convert string to base64 in sql server, which is a follows,

ALTER FUNCTION [dbo].[uFnStrToB64]
(
    @InputString VARCHAR(MAX)
)
RETURNS VARCHAR(MAX)
AS
BEGIN
    RETURN (
        SELECT
            CAST(N'' AS XML).value(
                  'xs:base64Binary(xs:hexBinary(sql:column("bin")))'
                , 'VARCHAR(MAX)'
            )
        FROM (
            SELECT CAST(@InputString AS VARBINARY(MAX)) AS bin
        ) AS RetVal
    )
END;

Now, I'm calling this function in following stored procedure, as follows

ALTER PROCEDURE [dbo].[UpdateTableColumnWithB64]
    @tbl sysname,
    @Name sysname
AS
BEGIN
 SET NOCOUNT ON;
 DECLARE @SQL NVARCHAR(MAX);

  SET @SQL =  N' UPDATE ' + QUOTENAME(@tbl)
            + N' SET ' + @Name + ' =  ' + dbo.uFnStrToB64(@Name)
 EXECUTE sp_executesql @SQL
END

But instead of updating the column value with base 64 of the column value, it is replacing it with base 64 of column name. But when I run following update command, it works flawlessly,

UPDATE mytable SET mycolumn=dbo.uFnStrToB64(mycolumn)

I'm calling stored proc as follows,

DECLARE @return_value int

EXEC    @return_value = [dbo].[UpdateTableColumnWithB64]
        @tbl = mytable,
        @Name = mycolumn

SELECT  'Return Value' = @return_value

GO

Adding create and insert table commands, if someone wants to run it and try it,

CREATE TABLE mytable(
    mycolumn varchar(max) NULL
);

INSERT INTO mytable(mycolumn) VALUES ('test'), ('test2'), ('test3');

Can somebody help me understand, when I move the update statement to stored procedure why the same update statement takes mycolumn as string instead of getting value from column? I'm open to change function and stored proc Or open for ways to do base 64 conversion in stored proc without usage of function.

Answer 1

Thanks to @dan, following is the solution

SET @SQL = N' UPDATE ' + QUOTENAME(@tbl) 
+ N' SET ' + QUOTENAME(@Name) 
+ N' = dbo.uFnStrToB64(' + QUOTENAME(@Name) 
+ N');';