Question

I omit the steps before the last one. I get the table like: year, month, postid, clicks. How do I get top-k postid for each year, month group?

For example, I have data:

2012,1,a,5
2012,1,b,3
2012,1,c,8
2012,2,a,2
2012,2,c,5
2012,2,d,6

Assuming k=2, I want the results like:

2012,1,c,8
2012,1,a,5
2012,2,d,6
2012,2,c,5

Answer 1

Give this a try:

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 here