LeetCode（数据库）- 页面推荐Ⅱ

WITH t AS(SELECT user1_id user_id FROM Friendship
UNION 
SELECT user2_id FROM Friendship),
tt AS(SELECT user_id, user2_id friend_id FROM t JOIN Friendship ON user_id = user1_id 
UNION
SELECT user_id, user1_id FROM t JOIN Friendship ON user_id = user2_id)
SELECT tt.user_id,  l1.page_id, COUNT(*) friends_likes
FROM tt JOIN Likes l1 ON friend_id = l1.user_id  LEFT JOIN Likes l2 ON l1.page_id = l2.page_id AND l2.user_id = tt.user_id
WHERE l2.user_id IS NULL # 最后一个筛选自身已喜欢的 page 采用表连接, 不能用子查询, 否则超时
GROUP BY tt.user_id, l1.page_id