多表 MySQL 查询中的重复行

Question

我们有三个表，“cameras”、“cameratypes”和“locations”。

这里每个都有样本数据。

位置（location_id、user_id、contact1）

select * from locations;
+-------------+---------+--------------------+
| location_id | user_id | contact1           |
+-------------+---------+--------------------+
|          38 |      96 | Joe Blow           |
|          42 |      96 | Joe Blow           |
|          45 |     105 | Francis McLucky    |
|          50 |     113 | Ogre L'Bosch       |
|          53 |     113 | Ogre L'Bosch       |
+-------------+---------+--------------------+

相机类型（相机类型 ID，标题）

select * from cameratypes;
+---------------+----------------+
| cameratype_id | title          |
+---------------+----------------+
|             1 | Pan Tilt Zoom  |
|             2 | Fixed          |
|             3 | Mobile         |
+---------------+----------------+

相机(camera_id, cameratype_id, location_id, user_id)

select * from cameras
+-----------+---------------+-------------+---------+
| camera_id | cameratype_id | location_id | user_id |
+-----------+---------------+-------------+---------+
|      4633 |             3 |          38 |      96 |
|      4637 |             3 |          38 |      96 |
|      4644 |             1 |          38 |      96 |
|      4962 |             1 |          45 |     105 |
|      4667 |             2 |          45 |     105 |
|      4681 |             2 |          50 |     113 |
|      4689 |             2 |          50 |     113 |
|      4684 |             2 |          50 |     113 |
|      4682 |             3 |          50 |     113 |
|      4691 |             2 |          53 |     113 |
+-----------+---------------+-------------+---------+

我正在尝试将表格加入报告中，其中输出是摄像机列表，带有所有者的姓名和摄像机类型。

然而，当我运行查询时，我得到了这个结果（基本上是将用户 113 的摄像头数量（他有五个）乘以他拥有的摄像头位置数量（即两个）。

我的查询是：

SELECT 
    loc.contact1 AS 'Camera Owner Name', 
    loc.location_id AS 'Location ID', 
    cam.camera_id AS 'Camera ID', 
    camtype.title AS 'Camera Type' 
FROM engine4_securonet_cameras AS cam 
LEFT JOIN engine4_securonet_locations AS loc ON cam.user_id = loc.user_id 
JOIN engine4_securonet_cameratypes AS camtype ON cam.cameratype_id = camtype.cameratype_id 
WHERE loc.user_id = 113 
ORDER BY cam.camera_id;

查询的结果是：

+-------------------+-------------+-----------+--------+
| Camera Owner Name | Location ID | Camera ID | Type   |
+-------------------+-------------+-----------+--------+
| Alexis Neve       |          50 |      4681 | Fixed  |
| Alexis Neve       |          53 |      4681 | Fixed  |
| Alexis Neve       |          50 |      4682 | Mobile |
| Alexis Neve       |          53 |      4682 | Mobile |
| Alexis Neve       |          50 |      4684 | Fixed  |
| Alexis Neve       |          53 |      4684 | Fixed  |
| Alexis Neve       |          50 |      4689 | Fixed  |
| Alexis Neve       |          53 |      4689 | Fixed  |
| Alexis Neve       |          50 |      4691 | Fixed  |
| Alexis Neve       |          53 |      4691 | Fixed  |
+-------------------+-------------+-----------+--------+
10 rows in set (0.00 sec)

如何消除这种重复？非常感谢！

Answer 1

您正在加入与所有者关联的所有位置，而不仅仅是相机的位置。

SELECT
  loc.contact1 as Owner,
  loc.location_id as "Location Id", 
  cam.camera_id AS "Camera ID", 
  camtype.title AS "Camera Type"
FROM cameras AS cam

LEFT JOIN locations AS loc 
       ON loc.location_id = cam.location_id
      AND cam.user_id = loc.user_id                                                            

JOIN cameratypes AS camtype 
  ON cam.cameratype_id = camtype.cameratype_id

WHERE loc.user_id = 113 
ORDER BY cam.camera_id;