需要简单加入的帮助

Question

爱

问题的权利。

SELECT *,t.id AS threadid FROM threads t 
LEFT JOIN players p on p.id = t.last_poster 
WHERE t.boardid = $boardid

我在线程中有两个字段叫做posterid和lastposterid 。 哪个是线程启动器/最后一张海报的ID。 我想做的是从玩家表中获取他们的名字。

但是怎么样？

Answer 1

你只需要两次加入你的玩家表，就像这样。

SELECT
    threads.*,
    starterPlayer.*,
    lastPosterPlayer.*
FROM
    threads
LEFT OUTER JOIN
    players starterPlayer
ON
    starterPlayer.id = threads.posterid
LEFT OUTER JOIN
    players lastPosterPlayer
ON
    lastPosterPlayer.id = threads.lastposterid

Answer 2

您可以两次加入同一个表，并为表提供不同的别名。

这假定总会有第一张和最后一张海报，如果是这种情况，那么你想要一个INNER JOIN而不是LEFT JOIN ，你需要更改select语句来获取相关字段。

SELECT t.id AS threadid, playerFirst.name AS FirstPoster, playerLast.name as LastPoster 
FROM threads t
INNER JOIN
   players playerFirst ON playerFirst.id = t.posterid
INNER JOIN 
   players playerLast ON playerLast.id = t.lastposterid

Answer 3

怎么样...

SELECT *,
    (SELECT name
         FROM players
         WHERE players.id = threads.posterid) AS poster,
    (SELECT name
         FROM players
         WHERE players.id = threads.lastposterid) AS last_poster
    FROM threads;