Summing multiple dynamically declared variables (fast) - sql

Question

I'm developing a program where I have multiple columns which I need to show the sum of to anEnd-user. These columns are assigned in the table ColumnTest in the column ColumnNames and their names can be changed by the user. Therefore I need to look up all the column names in ColumnTest\ColumnNames and afterwards sum all values regarding these columnnames from the outputtable.
I have previously used this script where I get all the columnnames in #cols like this [col1].[col2].[col3] and so on, but when I try to run the query I'm not able to sum these columns using '+ #cols + '. When I run this I get the following error: The SUM function requires 1 argument(s). Is there a viable option to do this procedure, without compromising the loading-time substantially?
DECLARE
#cols AS NVARCHAR(MAX),
#query AS NVARCHAR(MAX);
SELECT #cols = STUFF((SELECT distinct ',' + QUOTENAME(c.ColumnNames)
FROM ColumnTest c
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)') ,1,1,'')
set #query =
'
SELECT
ID
,SUM('+ #cols + ')
FROM Output_table
GROUP BY
ID
'
execute(#query);

Answer 1

Try this,
DECLARE
#cols AS NVARCHAR(MAX)='',
#query AS NVARCHAR(MAX);
SELECT #cols += 'SUM('+ColumnNames+') as ['+ColumnNames+'],'
from
(
SELECT distinct ColumnNames FROM ColumnTest
)A
SELECT #cols=LEFT(#cols,LEN(#cols)-1)
set #query =
'
SELECT ID,'+ #cols + '
FROM Output_table
GROUP BY ID
'
execute(#query);
or if you want addition of all column values you can use below query
DECLARE #cols AS NVARCHAR(MAX)='',
#query AS NVARCHAR(MAX);
SELECT #cols += ''+ColumnNames+'+'
FROM
(
SELECT DISTINCT ColumnNames FROM ColumnTest
)A
SELECT #cols=LEFT(#cols,LEN(#cols)-1)
set #query =
'
SELECT ID,SUM('+ #cols + ')
FROM Output_table
GROUP BY ID
'
execute(#query);

Answer 2

You can not use comma separated like ','
You can use '+'
Try this
DECLARE
#cols AS NVARCHAR(MAX),
#query AS NVARCHAR(MAX);
SELECT #cols = STUFF((SELECT distinct '+' + QUOTENAME(c.ColumnNames)
FROM ColumnTest c
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)') ,1,1,'')
set #query =
'
SELECT
ID
,SUM('+ #cols + ')
FROM Output_table
GROUP BY
ID
'
execute(#query);