Question

在过去的一个月里，我一直在提高自己的MYSQL技能，但是到了无法确定该怎么做的地步。

select id, fname,lname,status, email, country_name, login_count, 
        (select count(ID) from applications inner join players on users.id = players.user_id inner join applications on players.id = applications.user_id where players.id = applications.player_id),  (select count(ID) from messages where users.id = messages.user_id) AS '# of Messages', (select count(id) from participants where users.id = participants.user_id) AS '# of Threads', (SELECT created_at FROM messages where users.id = messages.user_id ORDER BY id DESC LIMIT 1 ) AS 'Last Message Date', updated_at
from users
where type = 'player'
#And (country_name = 'australia' or country_name = 'new zealand' or country_name = 'South Africa')
and status = 'active'
and (select count(ID) from messages where users.id = messages.user_id) = 0
and (select count(id) from participants where users.id = participants.user_id) != 0
and updated_at > '2018-10-01'
order by updated_at desc

我正在尝试获取播放器类型用户创建的应用程序数量。我还渴望获得它们所涉及的消息和线程的数量。

如果删除

，我可以很好地运行查询

（从用户上的应用程序内部加入玩家中选择count（ID）。id= players.user_id玩家上的内部加入玩家中的应用程序。id= applications.user_id，其中players.id = Applications.player_id），

但是一旦我尝试获取应用程序的数量，它就会返回此

不是唯一的表/别名：“应用程序”

任何帮助将不胜感激。

（请客气，我是在自学SQL）

Answer 1

您的问题是当您在此处运行子查询时：

select count(ID)
from applications
inner join players on users.id = players.user_id
inner join applications on players.id = applications.user_id
where players.id = applications.player_id

每次在给定查询中多次引用一个表时，都需要为该表引用提供唯一的别名。此查询两次触摸applications表，因此MySQL在给定的情况下不知道您要指的是哪个表（通常，无论如何，您都应该为表起别名，因为它使EXPLAIN语句易于调试，并使查询更具可读性。

您可以使用as语法添加别名：

select
    count(mainApps.ID)
from
    applications as mainApps
inner join
    players as appPlayers
    on  appPlayers.id = mainApps.playerId
inner join
    applications as joinedApps
    on  appPlayers.id = joinedApps.user_id
where
    users.id = appPlayers.user_id

尝试从多个mysql表中获取计数

1 个答案: