内部连接具有多行用途

Question

我需要从成员表中选择2行来获取名称和ID，我需要从消息表中获取所有行。这是我到目前为止任何提示或帮助表示赞赏 编辑 表

 SELECT member_messages.message_to, 
       member_messages.message_from, 
       member_messages.message_time, 
       member_messages.message_body,
       members.display_name,members.id 
FROM members
INNER JOIN member_messages 
   ON member_messages.message_to = members.id  AS 'to'
INNER JOIN member_messages 
   ON member_messages.message_from = members.id AS 'from'
WHERE member_messages.message_to = '$userId'

Answer 1

您的联接几乎是正确的，但您需要引用您在to列表中的from中指定的表别名JOIN和SELECT，而不是而不是直接提到members。由于TO和FROM都是MySQL reserved keywords，因此您必须同时引用它们。

SELECT member_messages.message_to, 
       member_messages.message_from, 
       member_messages.message_time, 
       member_messages.message_body,
       /* Select cols from both aliases of members, with column aliases */
       `to.display_name AS to_name,
       `to`.id AS to_id ,
       `from`.display_name AS from_name,
       `from`.id AS from_id 
/* First reference to members as `to` */
FROM members AS `to`
INNER JOIN member_messages 
   ON member_messages.message_to = `to`.id
/* Second reference to members as `from` */
INNER JOIN members AS `from` 
   ON member_messages.message_from = `from`.id
WHERE member_messages.message_to = '$userId'

Answer 2

如果你把db结构放在这里会有所帮助。假设您的表'member_messages'包含您想要阅读的所有消息，那么您需要将这两个消息与您的'members'表一起加入，以检索'from'和'to'成员的名称。

你可能正在寻找这样的东西：

SELECT
    message.message_time, message.message_body,
    member_to.id, member_to.display_name,
    member_from.id, member_from.display_name
FROM   
    member_messages AS message
JOIN   
    members AS member_to ON members.id = member_messages.message_to 
JOIN   
    members AS member_from ON members.id = members_messages.message_from

如果您的数据库系统中保留了某些单词，请更改它们。