https://www.postgresql.org/docs/current/tutorial-window.html

partition
    https://edu.postgrespro.ru/sqlprimer/sqlprimer-2019-msu-04.pdf
    ! p68
    select
        ...
        count(*) over (
            partition by date_trunc('month', b.book_date)
            order by b.book_date
        ) as count
    from
        ...
        join bookings b on ...
    ;
    over - necessary keyword (count function became a window-one)

sample
    select depname, empno, salary, avg(salary) over (partition by depname) from empsalary;

      depname  | empno | salary |          avg
    -----------+-------+--------+-----------------------
     develop   |    11 |   5200 | 5020.0000000000000000
     develop   |     7 |   4200 | 5020.0000000000000000
     develop   |     9 |   4500 | 5020.0000000000000000
     develop   |     8 |   6000 | 5020.0000000000000000
     develop   |    10 |   5200 | 5020.0000000000000000
     personnel |     5 |   3500 | 3700.0000000000000000
     personnel |     2 |   3900 | 3700.0000000000000000
     sales     |     3 |   4800 | 4866.6666666666666667
     sales     |     1 |   5000 | 4866.6666666666666667
     sales     |     4 |   4800 | 4866.6666666666666667
    (10 rows)