Cumulative Sum in MySQL - mysql

Question

Using MySQL. I want to get cumulative sum.
This is my table
CREATE TABLE `user_infos`
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
(..)
`created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
`updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
PRIMARY KEY (`id`) )
And what I want to get is
+-------+-------+----------------+
| month | count | cumulative_sum |
+-------+-------+----------------+
| 01 | 100 | 100 |
| 02 | 101 | 201 |
| ... | 110 | 311 |
| 12 | 200 | 511 |
+-------+-------+----------------+
but the result is
+-------+-------+----------------+
| month | count | cumulative_sum |
+-------+-------+----------------+
| 01 | 100 | 100 |
| 02 | 101 | 101 |
| ... | 110 | 110 |
| 12 | 200 | 200 |
+-------+-------+----------------+
This is my wrong query..
select
T1.Month,T1.Count,
#runnung_total := (#running_total + T1.Count) as cumulative_sum
from (
select date_format(created_at,'%m') as Month,count(1) as Count from users
where date_format(created_at,'%Y')='2016'
group by(date_format(created_at,'%m'))
union
select date_format(created_at,'%m') as Month,count(1) as Count from users
where date_format(created_at,'%Y')='2017'
group by(date_format(created_at,'%m')) ) as T1
join (select #running_total := 0) as R1;
I referred to this. What's wrong in my code?

Answer 1

You can achieve that in two steps: first of all get the sum for each year and month
select concat(year(created_at), lpad(month(created_at), 2, '0')) as ye_mo,
count(*) as cnt
from users
group by concat(year(created_at), lpad(month(created_at), 2, '0'))
Then join it with itself, having each row matched with all previous ones
select t1.ye_mo, sum(t2.cnt)
from (
select concat(year(created_at), lpad(month(created_at), 2, '0')) as ye_mo,
count(*) as cnt
from users
group by concat(year(created_at), lpad(month(created_at), 2, '0'))
) t1
join (
select concat(year(created_at), lpad(month(created_at), 2, '0')) as ye_mo,
count(*) as cnt
from users
group by concat(year(created_at), lpad(month(created_at), 2, '0'))
) t2
on t1.ye_mo >= t2.ye_mo
group by t1.ye_mo
order by t1.ye_mo
Edit
The query above assumes you want the running sum to increase across different years. If you want to display the months only, and aggregate the values of different years in the same month, you can change id this way
select t1.mnt, sum(t2.cnt)
from (
select month(created_at) as mnt,
count(*) as cnt
from userss
group by month(created_at)
) t1
join (
select month(created_at) as mnt,
count(*) as cnt
from userss
group by month(created_at)
) t2
on t1.mnt >= t2.mnt
group by t1.mnt
order by t1.mnt
Finally, if you want the running sum to reset at the beginning of each year, you can do that like this
select t1.yr, t1.mn, sum(t2.cnt)
from (
select year(created_at) as yr, month(created_at) as mn,
count(*) as cnt
from userss
group by year(created_at), month(created_at)
) t1
join (
select year(created_at) as yr, month(created_at) as mn,
count(*) as cnt
from userss
group by year(created_at), month(created_at)
) t2
on t1.yr = t2.yr and
t1.mn >= t2.mn
group by t1.yr, t1.mn
order by t1.yr, t1.mn
All three versions can be seen in action here

Answer 2

Variables are the right way to go. You can simplify your query:
select m.Month, m.cnt,
(#running_total := (#running_total + m.cnt) ) as cumulative_sum
from (select month(created_at) as Month, count(*) as cnt
from users
where year(created_at) in (2016, 2017)
group by month(created_at)
) m cross join
(select #running_total := 0) params
order by m.Month;

Answer 3

Starting with MySQL 8, the ideal approach to calculate cumulative sums is by using SQL standard window functions rather than the vendor-specific, and not stricly declarative approach of using local variables. Your query can be written as follows:
WITH data(month, count) AS (
SELECT date_format(create_at, '%m') AS month, count(*) AS count
FROM users
GROUP BY date_format(create_at, '%m')
)
SELECT
month,
count,
sum(count) OVER (ORDER BY month) AS cumulative_sum
FROM data