mysql - 复杂查询需要指导

Question

下面是系统的一些背景知识。用户的战斗和胜利者是赢得最多回合的人。我需要帮助才能加入三个表。我有一个表 user，它存储用户信息。存储匹配信息的表 match。表 rounds 存储每场比赛每轮的获胜者，因此如果一场比赛有 5 轮，则表 rounds 将有五行用于该场比赛，并将记录每场比赛的获胜者每轮。

这是一些示例数据:表用户:(userid为主键)

userid   username
-----------------
  1       Kevin
  2       Sam
  3       Steve
  4       Matt

表匹配:(id为主键。challenger和challenged都是user.userid的外键)

 id  challenger  challenged  rounds
-----------------------------------
 1      2           3          3
 2      1           2          1
 3      2           3          3
 4      2           4          1

表回合:(所有字段都是主键。id 是match.id 的外键，winner 是user.userid< 的外键)

 id  round  winner
------------------
 1    1       2
 1    2       2
 1    3       3
 2    1       1
 3    1       2
 3    2       3
 3    3       2
 4    1       4

我正在尝试构建一个将输出以下结果的查询:

winner  won   loser  won  
------------------------
 Sam     2    Steve   1
 Kevin   1    Sam     0
 Sam     2    Steve   1
 Matt    1    Sam     0

以上结果显示了每场比赛的赢家和输家。 won 字段分别显示获胜者和失败者在该场比赛中获胜的回合数。

有人知道如何构建上述查询吗？

Answer 1

select 
    case when w1 > w2 then u1 else u2 end as winner,
    case when w1 > w2 then w1 else w2 end as won,
    case when w1 > w2 then u2 else u1 end as loser,
    case when w1 > w2 then w2 else w1 end as won
from (
    select m.id, u1.username as u1, u2.username as u2, 
        count(r1.winner) as w1,
        count(r2.winner) as w2
    from match m
    join user u1 on m.challenger = u1.userid
    join user u2 on m.challenged = u2.userid
    join rounds r on r.id = m.id
    left join rounds r1 on r.id = r1.id and r.round = r1.round and r1.winner = u1.userid
    left join rounds r2 on r.id = r2.id and r.round = r2.round and r2.winner = u2.userid
    group by m.id, u1.username, u2.username
) t

它依赖于(Problems with NULL Values):

Aggregate (summary) functions such as COUNT(), MIN(), and SUM() ignore NULL values.

这是一个优势。