How to get the running total for data grouped by month for a MySQL query

```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