22. 친구 수 집계하기

with user_a_counts as (
	SELECT
	user_a_id AS user_id,
	COUNT(DISTINCT user_b_id) AS friend_count
	FROM edges
	GROUP BY user_a_id
),
user_b_counts as (
	SELECT
	user_b_id AS user_id,
	COUNT(DISTINCT user_a_id) AS friend_count
	FROM edges
	GROUP BY user_b_id
)
SELECT
	u.user_id
	, coalesce(a.friend_count, 0) + coalesce(b.friend_count, 0) num_friends
FROM users u
left join user_a_counts a on a.user_id = u.user_id
left join user_b_counts b on b.user_id = u.user_id
group by u.user_id
order by num_friends desc, u.user_id asc