Counting renewals during count of subscription sold - php

Question

I've a table in which write every subscription sold of my magazine, like this:
USERID | DATE |
31 | 2011-09-22 |
54 | 2011-09-22 |
59 | 2011-09-23 |
11 | 2011-10-02 |
88 | 2011-10-05 |
31 | 2011-10-06 |
17 | 2011-10-12 |
54 | 2011-10-15 |
31 | 2011-11-05 |
54 | 2011-11-12 |
Now, for statistical purpose, i need to having an outcome in which i see, for every single month, how many subscriptions i've sold and how many users have already bought once this.
For instance, if we look the datas on top, i should have an outcome likie this:
DATE | SOLD | RENEWAL
09 | 3 | 0
10 | 5 | 1
11 | 2 | 2
I can to group the subscriptions sold monthly, but i can't add the "renewal" info.
SELECT COUNT( * ) AS sold, MONTH(date) FROM table_sold WHERE date >= CURDATE() - INTERVAL 13 MONTH GROUP BY YEAR( date ) , MONTH( date ) ORDER BY date ASC LIMIT 1,15
In this way i only have an outcome like this:
DATE | SOLD
09 | 3
10 | 5
11 | 2
I've tried several option, with subquery, union and so on, but without successful.
There is to consider that the table has 70.000 entries and a query with a hard scan could be a problem.
In your opinion is there a way in mysql or i've to take the idea to make it in php?

Answer 1

Here is an idea. Look at the first date for each user. Then count renewals in every month that is not the first date:
select year(date), month(date), count(*) as sold,
sum(case when date <> firstdate then 1 else 0 end) as renewals
from subscriptions s join
(select userid, min(date) as firstdate
from subscriptions s
group by userid
) su
on s.userid = su.userid
group by year(date), month(date)
order by year(date), month(date);