title: Writing SQL query : getting the average value per minute
body: <p>I want to make a query getting the average created value per minute.
  I don't come up with any idea but using subquery like below.
  Would there be any other ways using only one query (without subquery)</p>
  <pre><code>with item_cnt_per_min as (
      select date_trunc('minute', created), count(*) as cnt
      from item
      where created &gt; '2020-09-21 06:10'
      group by 1
      order by cnt desc
  )
  select avg(cnt) from item_cnt_per_min
  </code></pre>