''' 
 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 monthly_users as (
     select 
          count(distinct(user_id)) as active_users,
          date_trunc('month', date) as month
     from 
         abakhytbekuly.users
     group by 
         date_trunc('month', date)
),
  change as (
      select active_users,
             month,
             lag(active_users) over(order by month) as prev_mau
      from 
          monthly_users
)
select 
    active_users, 
    month,
    prev_mau,
    100.0 * (active_users - prev_mau) / prev_mau as pct_change -- тут результат странный типа '-66.666666'
from 
     change
where 
     prev_mau is not null   
order by 
     month