Sql 需要简单连接的帮助吗

爱

对问题的正确认识

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，分别称为

posterid

和

lastposterid

。哪些是线程启动程序/最后一张海报的ID。我想做的是从球员表中得到他们的名字

---

但是怎么做呢？

你只需要加入你的玩家表两次，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

---

你只需要加入你的玩家表两次，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

那么

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;

那么

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;

---

您可以两次连接到同一个表，并为该表指定不同的别名

这假定始终存在第一个和最后一个海报，如果是这种情况，则需要

内部连接

而不是

左连接

，则需要更改select语句以获取相关字段

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

---

您可以两次连接到同一个表，并为该表指定不同的别名

这假定始终存在第一个和最后一个海报，如果是这种情况，则需要

内部连接

而不是

左连接

，则需要更改select语句以获取相关字段

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