fetching cumulative sum grouped by month on time interval - mysql

Question

This the users table. from where i am fetching data and filling in the missing months by adding table d to query.
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| uid | date_register |
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| 1 | 2011-07-20 02:24:36 |
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| 2 | 2012-10-03 07:37:43 |
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| ... | ... ... ... ... ... |
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| 300000 | 2015-12-19 04:13:51 |
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I want to fetch cumulative sum grouped by month. I am using following query to do this.
SELECT d.y, d.m, p.cum_sum
FROM
(SELECT a.y, b.m
FROM (SELECT 2015 AS Y UNION ALL SELECT 2014 AS Y) a
CROSS JOIN (SELECT 1 AS m UNION ALL SELECT 2 AS m UNION ALL SELECT 3 AS m UNION ALL SELECT 4 AS m UNION ALL SELECT 5 AS m UNION ALL SELECT 6 AS m UNION ALL SELECT 7 AS m UNION ALL SELECT 8 AS m UNION ALL SELECT 9 AS m UNION ALL SELECT 10 AS m UNION ALL SELECT 11 AS m UNION ALL SELECT 12 AS m) b
WHERE CONCAT(a.y, '-', b.m, '-01') BETWEEN CURDATE() - INTERVAL 13 MONTH AND CURDATE()
ORDER BY 1, 2) d
LEFT OUTER JOIN
(SELECT year1,
month,
#cnt := #cnt + total cum_sum
FROM (
SELECT YEAR(date_register) year1,
MONTH(date_register) month,
COUNT(*) total
FROM users
WHERE date_register >= DATE_FORMAT(NOW() ,'%Y-%m-01') - INTERVAL 1 YEAR
AND useractivated = 1
GROUP BY YEAR(date_register), MONTH(date_register)
) n, (SELECT #cnt := count(*) from users
where date_register< DATE_FORMAT(NOW() ,'%Y-%m-01') - INTERVAL 1 YEAR AND useractivated = 1) u)
p ON d.m=p.month AND d.y=p.year1
ORDER BY 1 ASC, 2 ASC
but it returns null value if there is no transactions in that particular month. like following (sample data)
+------+-------+---------+
| y | m | cum_sum |
+------+-------+---------+
| 2014 | 10 | 12356 |
| 2014 | 11 | 13567 |
| 2014 | 12 | 14239 |
| 2015 | 01 | 15234 |
| 2015 | 02 | 16571 |
| 2015 | 03 | NULL |
| 2015 | 04 | 24239 |
| 2015 | 05 | 34239 |
| 2015 | 06 | 44239 |
| 2015 | 07 | 45239 |
| 2015 | 08 | 46239 |
| 2015 | 09 | 57239 |
| 2015 | 10 | 67239 |
+------+-------+---------+
so there was no transaction in march 2015, so it returning null but it should return 16571 like previous row. What I am doing wrong? Thanks.

Answer 1

Try this:
select a.y, b.m,
(
SELECT count(*) as cum_count
FROM users
WHERE (YEAR(date_register) = a.y and MONTH(date_register) <= b.m) or year(date_register) < a.y
) as cum_count
from
(
SELECT 2015 AS Y UNION ALL SELECT 2014 AS Y
) a
CROSS JOIN
(
SELECT 1 AS m UNION ALL SELECT 2 AS m UNION ALL SELECT 3 AS m UNION ALL SELECT 4 AS m UNION ALL SELECT 5 AS m UNION ALL SELECT 6 AS m UNION ALL SELECT 7 AS m UNION ALL SELECT 8 AS m UNION ALL SELECT 9 AS m UNION ALL SELECT 10 AS m UNION ALL SELECT 11 AS m UNION ALL SELECT 12 AS m
) b
order by a.y, b.m