String aggregate using a variable

问题

Is is okay to use a variable to concatenate a value from several rows ( as an implicit aggregate function )? It seems to work fine on my machine, but I haven't seen it recommended.

declare @v_str varchar(4000) = ''
select top 5 @v_str = @v_str + ',' + city_name from city_table order by city_name
print @v_str

回答1:

From nvarchar concatenation / index / nvarchar(max) inexplicable behavior "The correct behavior for an aggregate concatenation query is undefined."

来源：https://stackoverflow.com/questions/44223465/string-aggregate-using-a-variable