```sql
SET @running_total:=0;
select
xx.*,
(@running_total := @running_total + xx.total) AS running_total
from (
select count(id) total, concat(concat(year(created_at), '-', lpad(month(created_at), 2, '0'))) year_month
from users
where created_at is not null group by year_month order by year_month
) as xx;
```
From