将一个表一列连接到另一个表多列

Question

我试过搜索了很多例子，但由于我有很多专栏，因此我有点复杂。这是一个简化版本：

用户

+--------+----------+
| userid | username |
+--------+----------+
| 1      | Tom      |
+--------+----------+
| 2      | Dick     |
+--------+----------+
| 3      | Harry    |
+--------+----------+

型

+--------+----------+
| typeid | typename |
+--------+----------+
| 1      | Desktop  |
+--------+----------+
| 2      | Laptop   |
+--------+----------+

计算机

+------+--------+-------------------+------------------+--------------------+--------------------+
| pcid | typeid | bought_by_user_id | assigned_user_id | created_by_user_id | updated_by_user_id |
+------+--------+-------------------+------------------+--------------------+--------------------+
| 1    | 2      | 1                 | 3                | 1                  | 3                  |
+------+--------+-------------------+------------------+--------------------+--------------------+
| 2    | 1      | 2                 | 2                | 2                  | 2                  |
+------+--------+-------------------+------------------+--------------------+--------------------+
| 3    | 1      | 3                 | 2                | 3                  | 1                  |
+------+--------+-------------------+------------------+--------------------+--------------------+

我想要实现的结果是

+------+---------+----------------+---------------+-----------------+-----------------+
| pcid | type    | bought_by_user | assigned_user | created_by_user | updated_by_user |
+------+---------+----------------+---------------+-----------------+-----------------+
| 1    | Desktop | Tom            | Harry         | Tom             | Harry           |
+------+---------+----------------+---------------+-----------------+-----------------+
| 2    | Laptop  | Dick           | Dick          | Dick            | Dick            |
+------+---------+----------------+---------------+-----------------+-----------------+
| 3    | Desktop | Harry          | Dick          | Harry           | Tom             |
+------+---------+----------------+---------------+-----------------+-----------------+

我尝试过使用多个ON：

SELECT * FROM computers
LEFT JOIN types ON computers.typeid = types.typeid
LEFT JOIN users
ON users.userid = computers.bought_by_user_id
ON users.userid = computers.assigned_user_id
ON users.userid = computers.created_by_user_id
ON users.userid = computers.updated_by_user_id 

但这不起作用...... Halp？

Answer 1

您需要在users表上多次加入：

SELECT 
    computers.pcid,
    types.typename,
    bought.username as bought_by_user,
    assigned.username as assigned_by_user,
    created.username as created_by_user,
    updated.username as updated_by_user
FROM computers
LEFT JOIN types ON computers.typeid = types.typeid
LEFT JOIN users bought ON computers.bought_by_user_id = bought.userid
LEFT JOIN users assigned ON computers.assigned_by_user_id = assigned.userid
LEFT JOIN users created ON computers.created_by_user_id = created.userid
LEFT JOIN users updated ON computers.updated_by_user_id = updated.userid

对于computers表中的四个用户列中的每一个，您都会单独加入用户表。每个连接表都有不同的别名，因此在select语句中，您可以访问连接表的任何列值。