The problem:

We may have several records belonging to a column or field that we want to concatenate and display in a single string. Something like this:

description
===========
value1
value2
value3

and we want to get something like this:

value1, value2, value3

that is, as if we had to transpose the column, but without complicating ourselves.

The solution is:

DECLARE @values VARCHAR(1000)
SELECT @values= COALESCE(@values + ', ', '') + description FROM folders
select @values as values

The magic is done by the COALESCE command that returns the expression that does not contain the NULL value among its arguments.

NOTE: This solution is valid only if no row is NULL.