Count distinct id between months previous year and same months current year Bigquery

I have a dataset in bigquery which contains order_date: DATE and customer_id.

order_date |    CustomerID
2020-01-01 |    111
2020-02-01 |    112
2020-03-01 |    111
2021-01-01 |    113
2021-02-01 |    115
2021-03-01 |    119

How can I count distinct customer_id between the months of the previous year and the same months of the current year? For example, from 2020-01-01 to 2021-01-01, then from 2020-02-01 to 2021-01-01, and so on until the current date and should be grouped by the latest date. The output looks like

order_date| count distinct CustomerID
2021-01-01| 5191
2021-02-01| 4859
2021-03-01| 3567
..........| ....

and the next periods shouldn’t include the previous.

Thanks in advance.

Answer

If you want just a count for each month you can expand the data and aggregate:

select mon, count(distinct customerid)
from t cross join
     unnest(generate_date_array(t.order_date, date_add(t.order_date, interval 11 month), interval 1 month)) mon
group by mon
order by mon;