Question

我收到了这段代码：

        SELECT
            topics.id,
            topics.id_first,
                            posts.id_first
          FROM topics
        LEFT
          JOIN posts
            ON posts.id = topics.id_first_msg

我的意图是做这样的事情：

        SELECT
            topics.id,
            topics.id_first,
                            posts.id_first,
                            posts.id_last
          FROM topics
        LEFT
          JOIN posts
            ON posts.id = topics.id_first_msg
        LEFT
          JOIN posts
            ON posts.id = topics.id_last_msg

但是，当我尝试做两次Left Join时，我收到一个错误。那是正确的方法呢？感谢。

Answer 1

您需要为多次加入的表提供别名：

SELECT
    topics.id,
    topics.id_first,
    p1.id_first,
    p2.id_last
FROM topics
LEFT JOIN posts p1 ON p1.id = topics.id_first_msg
LEFT JOIN posts p2 ON p2.id = topics.id_last_msg

Answer 2

您必须为第二个左连接添加别名;

    SELECT
        topics.id,
        topics.id_first,
                        posts.id_first,
                        posts2.id_last
      FROM topics
    LEFT
      JOIN posts
        ON posts.id = topics.id_first_msg
    LEFT
      JOIN posts AS posts2
        ON posts2.id = topics.id_last_msg

Answer 3

帖子表格不明确。首先 - 你真的需要加入两次吗？你能不能这样做：

    SELECT
        topics.id,
        topics.id_first,
        posts.id_first,
        posts.id_last,
        posts.id_first,
        posts.id_last
      FROM topics
    LEFT
      JOIN posts
        ON posts.id = topics.id_first_msg

第二 - 如果你真的必须加入两次 - 给post表两个不同的别名（你仍然需要编辑选定的列）

如何连接两次到另一张桌子？

3 个答案: