Zkuste to:
select postid, clicks,
@num := if(@year = @year and @month = month, @num + 1, 1) row_number,
@year := year year, @month := month month
from (
select * from t
order by year, month, clicks desc
) s, (select @num := 0, @year := '', @month := '') init
group by year, month, postid, clicks
having row_number <= 2
order by year, month, clicks desc
Fiddle zde