内部联接？ userid与其他userid的两个表连接

Question

我遇到了SQL语句的问题。无法理解MySQL中的多表交互。

所以我有两张桌子：

这是我的users表格：

这是我的messages表格：

我想要做的是生成一个基本上显示的表：

如果我是用户1

Daniel - Melbourne University

如果我向他发送了一条消息，或者从他那里收到了一条消息，我只希望他的名字出现。

完全失去了如何做到这一点，任何想法？

非常感谢！

Answer 1

有几种方法。

SELECT
  users.*
FROM
  users
INNER JOIN
(
  SELECT DISTINCT messages_senderid   AS user_id FROM messages WHERE messages_receiverid = 1
  UNION
  SELECT DISTINCT messages_receiverid AS user_id FROM messages WHERE messages_senderid   = 1
)
  contacts
    ON users.users_userid = contacts.user_id

或许......

SELECT
  users.*
FROM
  users
WHERE
     EXISTS (SELECT * FROM messages WHERE messages_senderid   = users.users_userid AND messages_receiverid = 1)
  OR EXISTS (SELECT * FROM messages WHERE messages_receiverid = users.users_userid AND messages_senderid   = 1)

Answer 2

SELECT users_name,users_university
FROM users
WHERE users_userid IN (SELECT messages_senderid 
                       FROM messages WHERE messages_receiverid=1)
     OR  users_userid IN (SELECT messages_receiverid 
                       FROM messages WHERE messages_senderid=1)

另一种方法

SELECT users_name,users_university
FROM users
WHERE users_userid IN ( SELECT 
                              CASE WHEN messages_receiverid=1 THEN messages_senderid
                                   WHEN messages_senderid=1 THEN messages_receiverid
                              END
                        FROM messages
                       )

Answer 3

试试这个：

select u.users_name, u.users_university
from users u
where u.users_usersid in(select m.messages_receiverid
from messages m
where m.messages_senderid=1
union
select m.messages_senderid
from messages m
where m.messages_receiverid=1)

Answer 4

假设:user_id是给定参数（在您的示例中：1），您可以尝试：

select concat(users_name, ' - ', users_university, ' University') 
from users u 
where exists (select 1 
              from messages 
              where messages_receiverid = :user_id 
                    or messages_senderid = :user_id)

在这种情况下，在子查询中组合exists和select 1是一种快速解决方案。