假设我有两个表玩家和统计数据,其中包含以下内容:
mysql> select * from players;
+----+-------+
| id | alive |
+----+-------+
| 1 | 0 |
| 2 | 1 |
+----+-------+
mysql> select * from stats;
+--------+------+------+-------+
| player | win | lose | ratio |
+--------+------+------+-------+
| 1 | 12 | 20 | 0.6 |
| 2 | 8 | 1 | 8 |
+--------+------+------+-------+
而且我想增加每个玩家的胜利计数器,同时还要更新他们的赢/输比率.这看起来像这样:
update `stats` set `win` = `win` + 1, `ratio` = `win` / `lose`;
请注意,增加的win值用于计算比率(正如mysql的手册中所述:1.8.2.2 UPDATE Differences).
现在,当一个JOIN添加到UPDATE查询以限制它仅更新alive = 1的播放器时,此行为会更改:
update `stats` st
inner join `players` pl
on ( pl.`id` = st.`player` )
set `win` = `win` + 1, `ratio` = `win` / `lose`
where pl.`alive` = 1;
mysql> select * from stats;
+--------+------+------+-------+
| player | win | lose | ratio |
+--------+------+------+-------+
| 1 | 12 | 20 | 0.6 |
| 2 | 9 | 1 | 8 |
+--------+------+------+-------+
我发现的唯一解决方案是将win的新值分配给临时变量,并在计算比率时使用该值:
update `stats` st
inner join `players` pl
on ( pl.`id` = st.`player` )
set
`win` = @tmpWin := ( `win` + 1 ),
`ratio` = @tmpWin / `lose`
where pl.`alive` = 1;
为什么MySQL表现得那样,是否有更优雅的解决方案解决这类问题(除了创建一个视图来动态计算比率)?
表格创建如下:
create table `players` (
`id` INT,
`alive` TINYINT,
primary key (`id`)
);
create table `stats` (
`player` INT,
`win` INT,
`lose` INT,
`ratio` FLOAT,
primary key (`player`)
);
我正在使用MySQL v5.7.17
最佳答案 我无法解释连接案例的行为确实看起来很奇怪,但以下工作符合预期:
UPDATE `stats`
SET `win` = `win` + 1, `ratio` = `win` / `lose`
WHERE player IN (SELECT id FROM players WHERE alive=1);