Danny Angus: Note to self, how to get row numbers in a MySQL SELECT

I've often wondered, idly, how to get MySQL to output a row number column, which would rank sorted and grouped results.
Well thanks to this excellent post I've found out how:

SELECT *, @rownum:=@rownum+1 as rank FROM ( 
-- Your Query Goes in Here
        SELECT t.*
        FROM table t
        WHERE something
        group by 2,3,4
        order by 1 desc limit 20
    ) day_rank, (SELECT @rownum:=0) r;