mysql - 用作子查询时产生不同结果的查询

Question

我对这个有点迷糊了——也许我用这个太多了以至于看不到解决方案？我有一个应用程序可以处理给定游戏每个赛季的玩家注册。我的目标是构建一个图表，显示每个节目每个赛季有多少新球员(一个节目实际上是年龄组)。

此查询是我当前用于执行任务的查询。这个想法是提取过去 5 个赛季的数据:

select
    `seasons`.*,
    (select COUNT(DISTINCT players.id) from `players`
        where exists (
            select * from `groups`
            inner join `player_season` on `groups`.`id` = `player_season`.`group_id`
            where `players`.`id` = `player_season`.`player_id` and
                `program_id` = 2 and
                `season_id` = seasons.id and
                `paid` is not null and
                `player_season`.`inactive` is null
        ) and not exists (
            select * from `seasons`
            inner join `player_season` on `seasons`.`id` = `player_season`.`season_id`
            where `players`.`id` = `player_season`.`player_id` and
                `seasons`.`id` < seasons.id
        )
    ) as new_players
from `seasons` order by `seasons`.`id` desc limit 5

您可以在这个数据库 fiddle 上看到该查询正在运行:https://www.db-fiddle.com/f/k13fnyhecyeGoTKUJvtum9/0 .它为 seasonId 2 拉出 2 的玩家数。

当我将它投入生产时，它显示的数字太高了，似乎显然是在吸引玩家，这是不应该的。因此，我删除了动态季节并将 seasonId 硬编码为 2，我只得到 1 个玩家而不是 2 个玩家，如这个 db fiddle 所示:https://www.db-fiddle.com/f/7k998nxexxY5K85a5ppXJN/0 .对于历史上下文，这里是单季查询:

select
    *
from `players`
where exists (
    select *
    from `groups`
    inner join `player_season` on `groups`.`id` = `player_season`.`group_id`
    where `players`.`id` = `player_season`.`player_id` and
        `program_id` = 2 and
        `season_id` = 2 and
        `paid` is not null and
        `player_season`.`inactive` is null
) and not exists (
    select *
    from `seasons`
    inner join `player_season` on `seasons`.`id` = `player_season`.`season_id`
    where `players`.`id` = `player_season`.`player_id` and
        `seasons`.`id` < 2
)

我逐行检查了这个查询，没有发现任何可能导致这种差异的差异。两个 db-fiddles 中的示例数据完全相同。 为什么这个查询用作子查询时会产生不同的结果？

Answer 1

查询在主选择和子查询的第二个条件中使用了“季节”表两次。

使用 alias区分 2 个表引用:

select
    `s1`.*,
    (select COUNT(DISTINCT players.id) from `players`
        where exists (
            select * from `groups`
            inner join `player_season` on `groups`.`id` = `player_season`.`group_id`
            where `players`.`id` = `player_season`.`player_id` and
                `program_id` = 2 and
                `season_id` = s1.id and
                `paid` is not null and
                `player_season`.`inactive` is null
        ) and not exists (
            select * from `seasons` AS s2
            inner join `player_season` on `s2`.`id` = `player_season`.`season_id`
            where `players`.`id` = `player_season`.`player_id` and
                `s2`.`id` < s1.id
        )
    ) as new_players
from `seasons` s1 order by `s1`.`id` desc limit 5