mysql group by month with custom starting day - mysql

Question

Is there a way in mysql to group by month, but with custom starting dates.
Say I want to count logins in a monthly basis, but with the condition that the month starts when a user register.
So for example user A registered on January 30th and user B on January 15th
I should group the logins as follow:
* User A: January 30th - February 28th, March 1st - March 30th, March 31 - April 30 and so on and so forth
* User B: January 15th - February 14th, February 15th - March 14th and so on and so forth
I guess I need to use something like DATE_ADD('2013-01-30', INTERVAL 1 MONTH); but I can not seem to find a way to make the grouping.
UPDATE
#GarethD: You are right that was a typo
In general the month should start at the same day of the next month or the last day of the next month in case that the first is not possible, so if you registered in day 31, the month period would start in day 30 for months that does not have 31 days and the last day of February either 28 or 29
Example:
Given that
id 1 registered on 2012-12-16
id 2 registered on 2013-01-29
and the following table
+----+------------+
| id | date |
+----+------------+
| 1 | 2013-01-15 |
| 1 | 2013-01-16 |
| 1 | 2013-01-17 |
| 1 | 2013-01-17 |
| 2 | 2013-03-20 |
| 2 | 2013-03-21 |
| 2 | 2013-03-28 |
| 2 | 2013-03-29 |
| 2 | 2013-03-30 |
+----+------------+
the results should be
+----+----------------------------+-------+
| id | range | count |
+----+----------------------------+-------+
| 1 | 2012-12-16, 2013-01-15 | 1 |
| 1 | 2013-01-16, 2013-02-15 | 3 |
| 2 | 2013-02-2[8|9], 2013-03-28 | 3 |
| 2 | 2013-03-29, 2013-04-28 | 2 |
+----+----------------------------+-------+
I hope the intent is clearer now.

Answer 1

For the following I am assuming you already have a numbers table, If you don't have a numbers table, then I'd recommend you make one then, but if you don't want to then you can create a number list on the fly
You can get a list of all boundaries by cross joining your userID and registered dates with your numbers table:
SELECT u.ID,
DATE_ADD(RegisteredDate, INTERVAL n.Number MONTH) PeriodStart,
DATE_ADD(RegisteredDate, INTERVAL n.Number + 1 MONTH) PeriodEnd
FROM User u
CROSS JOIN Numbers n;
This gives a table like:
ID PERIODSTART PERIODEND
1 2012-12-16 2012-12-16
2 2013-01-29 2013-01-29
1 2013-01-16 2013-01-16
2 2013-02-28 2013-02-28
Example on SQL Fiddle
You then need to join this to your main table, and do the count:
SELECT u.ID,
u.PeriodStart,
DATE_ADD(PeriodEnd, INTERVAL -1 DAY) PeriodEnd,
COUNT(*) AS `COUNT`
FROM ( SELECT u.ID,
DATE_ADD(RegisteredDate, INTERVAL n.Number MONTH) PeriodStart,
DATE_ADD(RegisteredDate, INTERVAL n.Number + 1 MONTH) PeriodEnd
FROM User u
CROSS JOIN Numbers n
) u
INNER JOIN T
ON T.ID = u.ID
AND T.Date >= u.PeriodStart
AND T.Date < PeriodEnd
GROUP BY u.ID, u.PeriodStart, u.PeriodEnd;
Giving a final result of:
ID PERIODSTART PERIODEND COUNT
1 2012-12-16 2013-01-15 1
1 2013-01-16 2013-02-15 3
2 2013-02-28 2013-03-28 3
2 2013-03-29 2013-04-28 2
Full Example on SQL-Fiddle
You can obviously concatenate your period start and end dates to make a 'range' string, but this is probably best handled in your application layer.
EDIT
This can be achieved with no subqueries which is likely to perform better:
SELECT u.ID,
DATE_ADD(u.RegisteredDate, INTERVAL n.Number MONTH) PeriodStart,
DATE_ADD(DATE_ADD(u.RegisteredDate, INTERVAL n.Number + 1 MONTH), INTERVAL -1 DAY) PeriodEnd,
COUNT(*) AS `COUNT`
FROM User u
CROSS JOIN Numbers n
INNER JOIN T
ON T.ID = u.ID
AND T.Date >= DATE_ADD(u.RegisteredDate, INTERVAL n.Number MONTH)
AND T.Date < DATE_ADD(u.RegisteredDate, INTERVAL n.Number + 1 MONTH)
GROUP BY u.ID, u.RegisteredDate, n.Number;
Example with no subquery on SQL-Fiddle
EDIT 2
This will get you all periods for all users up until the current period (i.e. where today falls within the date range)
SELECT u.ID,
DATE_ADD(u.RegisteredDate, INTERVAL n.Number MONTH) PeriodStart,
DATE_ADD(DATE_ADD(u.RegisteredDate, INTERVAL n.Number + 1 MONTH), INTERVAL -1 DAY) PeriodEnd,
COUNT(T.ID) AS `COUNT`
FROM User u
CROSS JOIN Numbers n
LEFT JOIN T
ON T.ID = u.ID
AND T.Date >= DATE_ADD(u.RegisteredDate, INTERVAL n.Number MONTH)
AND T.Date < DATE_ADD(u.RegisteredDate, INTERVAL n.Number + 1 MONTH)
WHERE DATE_ADD(u.RegisteredDate, INTERVAL n.Number + 1 MONTH) <= CURRENT_TIMESTAMP
GROUP BY u.ID, u.RegisteredDate, n.Number;
Example on SQL Fiddle