mySQL计数发生与JOIN [英] mySQL count occurrences with JOIN

问题描述

我有一个用于我的活动系统的标签系统我想创建一个标签云。

I have a tagging system for my events system I would like to create a 'tag cloud'.

我有活动，可以有多个类别。

I have Events, which can have multiple 'categories'.

以下是表格结构：

**Event_Categories** (Stores Tags / Categories)
| id  | name      |
+-----------------+
+ 1   | sport     |
+ 2   | charity   |
+ 3   | other_tag |


**Events_Categories** (Linking Table)
| event_id  | event_category_id |
+-------------------------------+
+    1      |       1           |   
+    2      |       2           |   
+    3      |       1           |   
+    3      |       2           |   

摘要：

Event ID 1 -> Sport
Event ID 2 -> Charity
Event ID 3 -> Sport, Charity

我想返回以下内容：

| tag_name  | occurrences |
+-----------+-------------+
|  sport    |     2       |
|  society  |     2       |

other_tag - 实际上没有返回，因为它有0次出现

other_tag - Not actually returned, as it has 0 occurrences

谢谢！ ：）

推荐答案

这将有效：

SELECT c.name AS tag_name, COUNT(ec.event_id) AS occurrences 
FROM Event_Categories c 
INNER JOIN Events_Categories ec ON c.id = ec.event_category_id 
GROUP BY c.id

将 INNER JOIN 更改为 LEFT JOIN 如果您想包含0次出现的类别

change the INNER JOIN to LEFT JOIN if you want to include categories with 0 occurances

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