Question

我有3张桌子

表学生

名称
REGID

表老师

名称
REGID

表格消息

ID
MSG
senderid
receiverid
TS

我希望得到那些与某人联系的人的姓名和名单。通过以下查询，我可以使用他们的注册，但我无法添加left join，因为我一直收到错误：

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

UNION

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

这就是我要为开始做的事情：

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

您可以在left join子句中添加from：

SELECT . . .
FROM ((SELECT sender_id AS contact FROM messages WHERE receiver_id = '...')
      UNION
      (SELECT receiver_id AS contact FROM messages WHERE sender_id = '...')
     ) LEFT JOIN
     blah
     ON . . . ;

这是你在做什么？

Answer 2

您错过了已加入的内部查询的别名。此外，要拥有明确的查询联合，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;

不能将左连接与联合组合

2 个答案: