''' 
 Задача 1.

Таблица users

| user_id | date       |
| ---     | ---        |
| 1       | 2018-07-01 |
| 234     | 2018-07-02 |
| 3       | 2018-07-02 |
| 1       | 2018-07-02 |
| ...     | ...        |
| 234     | 2018-10-04 |

Нужно найти ежемесячное процентное изменение месячной аудитории активных пользователей.
'''

with cte1 as (
select count(distinct(user_id)) as mau,
date_trunc('month', date) as month
from abakhytbekuly.users
group by date_trunc('month', date)
),
cte2 as (
select mau,
month,
lag(mau) over(order by month) as prev_mau
from cte1
)
select mau, 
month,
prev_mau,
100.0 * (mau - prev_mau) / prev_mau as pct_change
from cte2   
order by month