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 |
+------------+--------------+---------------+
创建数据
CREATE TABLE Teams(
team_id INT,
team_name VARCHAR(20));
INSERT INTO Teams VALUE(10, 'Leetcode FC'),(20, 'NewYork FC'),
(30, 'Atlanta FC'),
(40, 'Chicago FC'),
(50, 'Toronto FC');
CREATE TABLE Matches(
match_id INT,
host_team INT,
guest_team INT,
host_goals INT,
guest_goals INT);
INSERT INTO Matches VALUE(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);
解答
先分别查主客队得到的分数
-- 主队的得分
SELECT M.`host_team`, IF(M.`host_goals`>M.`guest_goals`,3,IF(M.`host_goals`=M.`guest_goals`, 1, 0)) AS points
FROM Matches AS M;
-- 客队得分
SELECT M1.`guest_team`, IF(M1.`guest_goals`>M1.`host_goals`,3,IF(M1.`guest_goals`=M1.`host_goals`, 1, 0)) AS points
FROM Matches AS M1;
合并
(SELECT M.`host_team`, IF(M.`host_goals`>M.`guest_goals`,3,IF(M.`host_goals`=M.`guest_goals`, 1, 0)) AS points
FROM Matches AS M)
UNION ALL
(SELECT M1.`guest_team`, IF(M1.`guest_goals`>M1.`host_goals`,3,IF(M1.`guest_goals`=M1.`host_goals`, 1, 0)) AS points
FROM Matches AS M1);
对team分组 求出每队的得分和
SELECT all_point.team, SUM(all_point.points)
FROM ((SELECT M.`host_team` AS team, IF(M.`host_goals`>M.`guest_goals`,3,IF(M.`host_goals`=M.`guest_goals`, 1, 0)) AS points
FROM Matches AS M)
UNION ALL
(SELECT M1.`guest_team` AS team, IF(M1.`guest_goals`>M1.`host_goals`,3,IF(M1.`guest_goals`=M1.`host_goals`, 1, 0)) AS points
FROM Matches AS M1)) AS all_point
GROUP BY all_point.team;
和teams表合并 进行排序即可
SELECT T.`team_name`, IFNULL(team_point.num_points,0) AS num_points
FROM Teams AS T
LEFT JOIN (SELECT all_point.team, SUM(all_point.points) AS num_points
FROM ((SELECT M.`host_team` AS team, IF(M.`host_goals`>M.`guest_goals`,3,IF(M.`host_goals`=M.`guest_goals`, 1, 0)) AS points
FROM Matches AS M)
UNION ALL
(SELECT M1.`guest_team` AS team, IF(M1.`guest_goals`>M1.`host_goals`,3,IF(M1.`guest_goals`=M1.`host_goals`, 1, 0)) AS points
FROM Matches AS M1)) AS all_point
GROUP BY all_point.team) AS team_point
ON T.`team_id` = team_point.team
ORDER BY num_points DESC, T.`team_name` ASC;
别的解答
类似地
首先通过union all将比赛结果表，将对两队的结果，联合起来。如果五场比赛，相当于10个球队，将比赛表左右两边对调，就可以是五场比赛的十支球队结果都包含了。
然后通过teams表left join 使得左边肯定多于右边，判断通过on后边的条件对比，右边分组里的每一行通过对比判断积分，求和。
select t.team_id,t.team_name,
sum(case when m.host_goals > m.guest_goals then 3
when m.host_goals = m.guest_goals then 1
else 0 end) num_points
from teams t
left join
(select host_team, guest_team, host_goals, guest_goals
from matches
union all
select guest_team host_team, host_team guest_team, guest_goals host_goals, host_goals guest_goals
from matches) m
on m.host_team = t.team_id
group by t.team_id
order by num_points desc, t.team_id asc
这一种也很好理解
select t.team_id,t.team_name,sum(
CASE WHEN t.team_id= m.host_team and m.host_goals>m.guest_goals THEN 3
WHEN m.host_goals = m.guest_goals THEN 1
WHEN t.team_id = m.guest_team and m.guest_goals>m.host_goals THEN 3 ELSE 0 END
) as num_points
from Teams t
left join Matches m
on t.team_id=m.host_team or t.team_id=m.guest_team
group by t.team_id
order by num_points desc ,team_id asc