SQL,如何查询表中的多个外键？

Question

我有一个项目表,其中有两个用户外键(user_id和winner_user_id),一个用于项目所有者,一个用于项目的获胜者.就像是

+----------------+-------------------------+------+-----+---------+----------------+
| Field          | Type                    | Null | Key | Default | Extra          |
+----------------+-------------------------+------+-----+---------+----------------+
| project_id     | int(10) unsigned        | NO   | PRI | NULL    | auto_increment | 
| start_time     | datetime                | NO   |     | NULL    |                | 
| end_time       | datetime                | NO   |     | NULL    |                | 
| title          | varchar(60)             | NO   |     | NULL    |                | 
| description    | varchar(1000)           | NO   |     | NULL    |                | 
| user_id        | int(11)                 | NO   |     | NULL    |                | 
| winner_user_id | int(10) unsigned        | YES  |     | NULL    |                | 
| type           | enum('fixed','auction') | YES  |     | NULL    |                | 
| budget         | decimal(10,0)           | YES  |     | NULL    |                | 
+----------------+-------------------------+------+-----+---------+----------------+

现在我尝试在单个查询中获取有关项目和两个用户的数据的信息.

所以我制定了一个类似的查询

SELECT projects.project_id, projects.title, projects.start_time,
            projects.description, projects.user_id, projects.winner_user_id,
            users.username as owner, users.username as winner
        FROM projects,users   
        WHERE projects.user_id=users.user_id
        AND projects.winner_user_id=users.user_id

其中显然返回一个空集.真正的问题是如何引用这些不同的user_id.我甚至尝试使用AS关键字,然后使用我在同一个sql查询中创建的名称,但显然这不起作用.

最后要说清楚我想要的东西

+------------+-------------------------------------------------+---------------------+---------+----------------+--------------+--------------+
| project_id | title                                           | start_time          | user_id | winner_user_id | owner        | winner       |
+------------+-------------------------------------------------+---------------------+---------+----------------+--------------+--------------+
|          1 | CSS HTML Tableless expert for site redesign     | 2009-09-01 21:07:26 |       1 |              3 | mr X        | mr Y        | 
|          2 | High Quality Ecommerce 3-Page Design HTML & CSS | 2009-09-01 21:10:04 |       1 |              0 | mr X        | mr Z        | 

如何构造查询来处理这个问题？

提前致谢.

Answer 1

你很近,但你需要加入用户表两次,一次是在所有者身上,一次是在获胜者身上.使用表别名来区分这两者.

SELECT 
      projects.project_id
    , projects.title
    , projects.start_time
    , projects.description
    , projects.user_id
    , projects.winner_user_id
    , users.username as owner
    , winnerUser.username as winner
FROM projects
INNER 
    JOIN users
    ON projects.user_id=users.user_id
INNER 
    JOIN users winnerUser
    ON projects.winner_user_id=winnerUser.user_id