How to calculate average for every month from start from year in Presto’s SQL (Athena)?

Below is an example of the table data I have

| date       | value |
| 2020-01-01 |  20   |
| 2020-01-14 |  10   |
| 2020-02-02 |  30   |
| 2020-02-11 |  25   |
| 2020-02-25 |  25   |
| 2020-03-13 |  34   |
| 2020-03-21 |  10   |
| 2020-04-06 |  55   |
| 2020-04-07 |  11   |

I would like to generate a result set as below

| date       | value |  average                       |
| 2020-01-01 |  20   |  Jan average                   |
| 2020-01-14 |  10   |  Jan average                   |   
| 2020-02-02 |  30   |  Jan & Feb average             |
| 2020-02-11 |  25   |  Jan & Feb average             |
| 2020-02-25 |  25   |  Jan & Feb average             |
| 2020-03-13 |  34   |  Jan & Feb & Mar average       |
| 2020-03-21 |  10   |  Jan & Feb & Mar average       |
| 2020-04-06 |  55   |  Jan & Feb & Mar & Apr average |
| 2020-04-07 |  11   |  Jan & Feb & Mar & Apr average |

I tried to use window function OVER() and PARTITION() but I managed to get average on month by month rather than starting from the year.

Any suggestions, please.

Thanks

Answer

I think you want:

select 
    t.*,
    avg(value) over(
        partition by year(date)
        order by month(date)
    ) running_avg
from mytable t

This puts each year in a separate partition, and the order partition rows by month.