Number of different people discussing each pull request

select avg(num_users) as average_num_users_commenting_per_pull_request, project_name, url
from
	(
	select projects.id as project_id, projects.name as project_name, 
			projects.url as url, pull_requests.id as pull_request_id,
			count(distinct users.id) as num_users
		from pull_request_comments
			join pull_requests on pull_requests.id = pull_request_comments.pull_request_id
			join projects on projects.id = pull_requests.base_repo_id
			join users on pull_request_comments.user_id = users.id
	group by projects.id, pull_requests.id
	) as user_count
group by project_id