Get historical average and count of a value where a date could exist more than once

I have a table with multiple equal date entries and a value. I need a table that calculates the historical value and the count of entries per date. I want to use the data to create some charts in gnuplot/etc later.

Raw data:

   date     | value
------------+------
 2017-11-26 |    5
 2017-11-26 |    5
 2017-11-26 |    5
 2017-11-28 |   20
 2017-11-28 |    5
 2018-01-07 |  200
 2018-01-07 |    5
 2018-01-07 |   20
 2018-01-15 |    5
 2018-01-16 |   50

Output should be:

   date     |  avg   | count         manual calc explanation
------------+--------+-------       ---------------------------------------
 2017-11-26 |      5 |    3          (5+5+5) / 3 = 5
 2017-11-28 |      8 |    2          (5+5+5+20+5) / 5 = 8
 2018-01-07 | 33.125 |    3          (5+5+5+20+5+200+5+20) / 8 = 33.125
 2018-01-15 |     30 |    1          (5+5+5+20+5+200+5+20+5) / 9 = 30
 2018-01-16 |     32 |    1          (5+5+5+20+5+200+5+20+5+50) / 10 = 32

If it is not possible to calculate two different columns, I would be fine for the avg column. For counting only the dates I have the solution “SELECT DISTINCT date, COUNT(date) FROM table_name GROUP BY date ORDER BY date”

I played around with DISTINCTs, GROUP BYs, JOINs, etc, but I did not find any solution. I found some other articles on the web, but no one covers a case where a date is more than once listed in the table.

Answer

You want a running average (total value divided by total count up to the row). This is done with window functions.

select
  date,
  sum(sum_value) over (order by date) as running_sum,
  sum(cnt) over (order by date) as running_count,
  sum(sum_value) over (order by date) /
  sum(cnt) over (order by date) as running_average
from
(
  select date, sum(value) as sum_value, count(*) as cnt 
  from mytable
  group by date
) aggregated
order by date;

Demo: https://dbfiddle.uk/?rdbms=postgres_13&fiddle=fb13b63970cb096913a53075b8b5c8d7