How can I repeat this query over several months in MySQL? - mysql

Question

I have a membership list with over 1.2M members. People commonly subscribe, unsubscribe, and re-subscribe to the list. Often, I find myself needing to know which users were subscribed at a particular moment in time. I have a table called subscription_history, with this structure:
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| id | native key |
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| user_id | foreign key that joins the user table |
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| change_code | 1 or 2 for subscriptions, 4-7 for unsubscriptions |
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| created_at | date-time stamp when the change was made |
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Right now, if I want to know who was subscribed at a particular date in the past (March 31, 2012 in this example), I run this query:
SELECT user_id
FROM
(SELECT
user_id
, MAX(created_at) AS last_change_date
, change_code
FROM subscriptionhistory
WHERE DATE(created_at) <= '2012-03-31'
GROUP BY user_id
) AS last
WHERE change_code IN (1,2)
This finds each user's last subscription action before or on the target date, then returns the user if that action was a subscription. We then use that list of users to run various other queries, such as the average lifetime sales. This system works well, but only for one date at a time. If I wanted to know the average subscriber's lifetime sales for every month of the year, I would have to run this query 12 times, manually incrementing the date in the WHERE statement each time.
Now I want to create a version of this that can I can use for more than a single date... so that it could give me all users subscribed in January, then February, etc., and I could run average lifetime sales for subscribers in each month. I can't just do a GROUP BY for this, since someone who was a subscriber in March might have unsubscribed in April and re-subscribed in June. I suppose I could 12 UNION queries ... but was hoping for something a little more elegant!
A few limiting parameters: I only have read-only access to the database; I cannot change anything about the table structure or make temporary tables. I have to do this only in MySQL - because of the way our CRM works, I can't use Python or PHP to manipulate results. Any help would be greatly appreciated! Please let me know if I am not explaining this well. Thanks!

Answer 1

SELECT user_id, group_concat(date_format(created_at, '%Y-%m')) as ActiveMonth from
(SELECT user_id, created_at, change_code from Subscriptions WHERE
change_code in (1,2) order by 1,2,3) b
group by user_id
order by user_id, ActiveMonth desc
You can take the group_concat out and the group by and it should give you a row and active month for every user_id.
I created a SQLFiddle and changed the table name to subscriptions for ease of use.
http://sqlfiddle.com/#!2/6b2f2/14