MySQL Rolling Count of Rows - mysql

Question

I'm trying to do a rolling count of registration growth on a student website.
The query looks as follows:
SELECT COUNT(type) as student_count, MONTH(created_at) as month, YEAR(created_at) as year
FROM users
WHERE type = 'student'
GROUP BY MONTH(created_at), YEAR(created_at)
ORDER BY year, month
This produces the following output:
What I'm trying to achieve in the query is to keep adding up the student_counts from the previous rows.
So:
December 2014 should have 15 students
January 2015 should have 16 students
February 2015 should have 34 students
and so on...
Is this possible in SQL or is it better to do this when outputting the data in the code itself?

Answer 1

select *, #sum := #sum + student_count as sum
from
(
SELECT YEAR(created_at) as year,
MONTH(created_at) as month,
COUNT(type) as student_count
FROM users
WHERE type = 'student'
GROUP BY year, month
ORDER BY year, month
) tmp
CROSS JOIN (select #sum := 0) s

Answer 2

Try using with rollup
SELECT COUNT(type) as student_count, MONTH(created_at) as month, YEAR(created_at) as year
FROM users
WHERE type = 'student'
GROUP BY YEAR(created_at), MONTH(created_at) WITH ROLLUP

Answer 3

Try this:
SELECT #cumulative := 0;
SELECT #cumulative := #cumulative + student_count student_count,
month, year
FROM (
SELECT COUNT(type) as student_count,
MONTH(created_at) as month,
YEAR(created_at) as year
FROM users
WHERE type = 'student'
GROUP BY MONTH(created_at), YEAR(created_at)
) A ORDER BY year, month

Answer 4

One approach to handle this in MySQL uses a correlated subquery to find the running total.
SELECT DISTINCT
(SELECT COUNT(*) FROM users u2
WHERE DATE_FORMAT(u2.created_at, '%Y-%m') <=
DATE_FORMAT(u1.created_at, '%Y-%m')) AS student_count,
DATE_FORMAT(created_at, '%Y-%m') AS ym
FROM users u1
WHERE type = 'student'
ORDER BY DATE_FORMAT(created_at, '%Y-%m');
Demo
Not much to explain here, except that SELECT DISTINCT gives us each unique year-month value in the table as a single record. We then count all rows at that point in time or earlier to find the running total.