What is the most efficient way to concatenate strings in SQL Server?
If you're on SQL Server 2017+, there's a built-in function that's a bit simpler to use than a subquery with XML and STUFF, etc. 

You can use STRING_AGG.

SELECT p.OwnerUserId, STRING_AGG(p.Score, ', ') AS aggro
FROM dbo.Posts AS p
WHERE p.Score &gt; 100
GROUP BY p.OwnerUserId


SELECT p.OwnerUserId, STRING_AGG(p.Score, ', ')                        
                                 WITHIN GROUP (ORDER BY p.Score DESC) AS aggro_order
FROM dbo.Posts AS p
WHERE p.Score &gt; 100
GROUP BY p.OwnerUserId