mysql - 在 2 列上连接 mysql 表两次 = 1 列

Question

我有一个包含消息的数据库。消息存储在一个表中，用户信息存储在另一表中。在消息表中，有一个author_id列，它代表来自用户表的作者的user_id，还有所有消息列，还有一个to_address，它代表来自用户表的“u_”+ user_id的串联。有没有什么可以连接这两个表，以便它在author_id 和to_address 中显示用户名而不是ID。

我已经尝试过

SELECT  username, ..., username
  FROM msgs
    INNER JOIN users
      ON user_id=author_id AND concat("u_",user_id)=to_address;

有明显错误我尝试过使用子查询，例如

SELECT
  ( SELECT username 
      FROM users
        INNER JOIN msgs
          ON user_id=author_id
  ) AS "From",
  ( SELECT username
      FROM users
        INNER JOIN msgs
          ON CONCAT("u_",user_id)=to_address
  ) AS "To",
  ( SELECT timestamp(message_time) FROM msgs
  ) AS "Sent",
  ( SELECT message_subject FROM msgs
  ) AS "Subject",
  ( SELECT message_text AS "Message" FROM msgs
  ) AS "Message"

并得到“子查询返回超过 1 行”。有什么办法可以成功做到这一点吗？

Answer 1

听起来你想要这样的东西:

SELECT
    from_user.username AS "From",
    to_user.username AS "To",
    timestamp(msgs.message_time) AS "Sent",
    msgs.message_subject AS "Subject",
    msgs.message_text AS "Message"

FROM msgs

INNER JOIN users AS from_user
ON msgs.author_id = from_user.user_id

INNER JOIN users AS to_user
ON msgs.to_address = CONCAT("u_", to_user.user_id);

基本上，您将users 表连接到msgs 表两次，为表的每个实例赋予不同的名称和不同的连接条件。然后，您可以从 users 表的特定实例中选择特定列。