Creating a histogram in postgreSQL

histogram.sql

with course_buckets as (
  select
      course_slug,
      extract(days from open_for_enrolment_at - proposal_submitted_at) as days,
      width_bucket(extract(days from open_for_enrolment_at - proposal_submitted_at), 0, 200,20) as bucket 
    from course_creation_milestones
    where open_for_enrolment_at > current_timestamp - interval '3 months'
),

bucket_totals as (
  select
    count(*) as total,
    bucket
  from course_buckets
  group by bucket
)
  
select
  all_the_buckets.bucket,
  case
    when all_the_buckets.bucket <= 20
      then (all_the_buckets.bucket-1) * 10 || '-' || (all_the_buckets.bucket * 10)
    else '200+' 
    end as bucket_range,
  COALESCE(bucket_totals.total, 0) as "Number of courses"
from 
  generate_series(1,21) as all_the_buckets(bucket)
left outer join bucket_totals on (all_the_buckets.bucket = bucket_totals.bucket)
order by all_the_buckets.bucket