1. 题目

Table: Teams

+---------------+----------+
| Column Name   | Type     |
+---------------+----------+
| team_id       | int      |
| team_name     | varchar  |
+---------------+----------+
此表的主键是 team_id，表中的每一行都代表一支独立足球队。

Table: Matches

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| match_id      | int     |
| host_team     | int     |
| guest_team    | int     | 
| host_goals    | int     |
| guest_goals   | int     |
+---------------+---------+
此表的主键是 match_id，表中的每一行都代表一场已结束的比赛，
比赛的主客队分别由它们自己的 id 表示，
他们的进球由 host_goals 和 guest_goals 分别表示。积分规则如下：赢一场得三分；
平一场得一分；
输一场不得分。

写出一条SQL语句以查询每个队的 team_id，team_name 和 num_points。
结果根据 num_points 降序排序，如果有两队积分相同，那么这两队按 team_id 升序排序。

查询结果格式如下：

Teams table:
+-----------+--------------+
| team_id   | team_name    |
+-----------+--------------+
| 10        | Leetcode FC  |
| 20        | NewYork FC   |
| 30        | Atlanta FC   |
| 40        | Chicago FC   |
| 50        | Toronto FC   |
+-----------+--------------+Matches table:
+------------+--------------+---------------+-------------+--------------+
| match_id   | host_team    | guest_team    | host_goals  | guest_goals  |
+------------+--------------+---------------+-------------+--------------+
| 1          | 10           | 20            | 3           | 0            |
| 2          | 30           | 10            | 2           | 2            |
| 3          | 10           | 50            | 5           | 1            |
| 4          | 20           | 30            | 1           | 0            |
| 5          | 50           | 30            | 1           | 0            |
+------------+--------------+---------------+-------------+--------------+Result table:
+------------+--------------+---------------+
| team_id    | team_name    | num_points    |
+------------+--------------+---------------+
| 10         | Leetcode FC  | 7             |
| 20         | NewYork FC   | 3             |
| 50         | Toronto FC   | 3             |
| 30         | Atlanta FC   | 1             |
| 40         | Chicago FC   | 0             |
+------------+--------------+---------------+

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/team-scores-in-football-tournament
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

2. 解题

• 按照主队，客队分别计算完 合并，再group by，left join 队名表

# Write your MySQL query statement below
select Teams.team_id, Teams.team_name, ifnull(Scores.score, 0) num_points
from Teams left join
(select team, sum(score) scorefrom((select host_team team, (case when host_goals > guest_goals then 3when host_goals = guest_goals then 1else 0 end) scorefrom Matches)union all(select guest_team team, (case when host_goals > guest_goals then 0when host_goals = guest_goals then 1else 3 end) scorefrom Matches)) tempgroup by team
) Scores
on Teams.team_id = Scores.team
order by num_points desc, team_id

690 ms

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