MySQL，第二个 INNER JOIN答案

【问题标题】：MySQL, second INNER JOINMySQL，第二个 INNER JOIN
【发布时间】：2013-03-02 17:43:02
【问题描述】：

您好，我想用一对 INNER JOIN 进行查询，我的错在哪里？

SELECT job_tbl.id, accounts.username AS starter, accounts.username AS worker, job_tbl.comment, job_tbl.date, job_tbl.status

来自job_tbl

在 job_tbl.starter = accounts.id 上的 INNER JOIN 帐户

在 job_tbl.worker = accounts.id 上的 INNER JOIN 帐户

job_tbl 表在这里：

+----+---------+--------+---------+------+--------+
| id | starter | worker | comment | date | status |
+----+---------+--------+---------+------+--------+
| 1  |    1    |    3   |   qwe   | date |   10   |
+----+---------+--------+---------+------+--------+
| 2  |    2    |    1   |   qwe   | date |   10   |
+----+---------+--------+---------+------+--------+

此处的帐户表：

+----+------------+-----------+-------+
| id |  username  | extension | email |
+----+------------+-----------+-------+
| 1  |   Julia    |    100    | email |
+----+------------+-----------+-------+
| 2  |    Eve     |    101    | email |
+----+------------+-----------+-------+
| 3  |    Max     |    102    | email |
+----+------------+-----------+-------+

我想要的结果是：

+----+---------+--------+---------+------+--------+
| id | starter | worker | comment | date | status |
+----+---------+--------+---------+------+--------+
| 1  |  Julia  |   Max  |   qwe   | date |   10   |
+----+---------+--------+---------+------+--------+
| 2  |   Eve   |  Julia |   qwe   | date |   10   |
+----+---------+--------+---------+------+--------+

【问题讨论】：

标签： mysql

【解决方案1】：

您没有指定将哪个accounts 表实例用作启动器或工作器。试试这个：

SELECT job_tbl.id, job_tbl.description, Starter.username AS starter, Worker.username AS worker, job_tbl.comment, job_tbl.date, job_tbl.status
FROM job_tbl
INNER JOIN accounts AS Starter ON job_tbl.starter = Starter.id
INNER JOIN accounts AS Worker ON job_tbl.worker = Worker.id

【讨论】：

【解决方案2】：

问题是您要加入两个表，但未在表 accounts 上指定 ALIAS，从而导致状态不明确。

SELECT  a.*,
        b.username StarterName,
        c.userName WorkerName
FROM    job_tbl a
        INNER JOIN account b
            ON a.starter = b.id
        INNER JOIN account c
            ON a.worker = c.ID

如需进一步了解联接，请访问以下链接：

Visual Representation of SQL Joins

【讨论】：