mysql - 不能将左连接与联合结合

Question

我有3张 table

表学生

• 姓名
• 注册

表老师

• 姓名
• 注册

表格消息

• 编号
• 留言
• 发件人身份
• 接收ID
• ts

我想得到与某个人联系的人的姓名和注册表。通过以下查询，我可以获取他们的 regid，但我无法添加 left join，因为我总是收到错误消息:

(SELECT sender_id AS contact FROM messages WHERE receiver_id = '...')

UNION

(SELECT receiver_id AS contact FROM messages WHERE sender_id = '...')

这就是我开始尝试做的事情:

SELECT teachers.name,
       teachers.regid
FROM   teachers
       LEFT JOIN ((SELECT sender_id AS contact
                   FROM   messages
                   WHERE  receiver_id = '...')
                  UNION DISTINCT
                  (SELECT receiver_id AS contact
                   FROM   messages
                   WHERE  sender_id = '...')) 
         ON t.contact = teachers.regid; 

Answer 1

您缺少连接内部查询的别名。 此外，要具有不同的查询联合，您应该只使用 UNION 运算符:

SELECT teachers.name, teachers.regid 
FROM teachers 
LEFT JOIN ( (SELECT sender_id AS contact 
             FROM   messages 
             WHERE  receiver_id = '...') 
             UNION 
            (SELECT receiver_id AS contact 
             FROM   messages 
             WHERE  sender_id = '...') ) t -- here!
ON t.contact = teachers.regid;