如何使用 sql 语句查找每个类别的产品数量

Question

我希望创建一个 sql 查询来列出属于每个电影类别的电影总数。 无法创建任何查询。 任何帮助将不胜感激。

+-------------+-------------+---------------------+
| category_id | name        | last_update         |
+-------------+-------------+---------------------+
|           1 | Action      | 2006-02-15 04:46:27 |
|           2 | Animation   | 2006-02-15 04:46:27 |
|           3 | Children    | 2006-02-15 04:46:27 |
|           4 | Classics    | 2006-02-15 04:46:27 |
|           5 | Comedy      | 2006-02-15 04:46:27 |
|           6 | Documentary | 2006-02-15 04:46:27 |
|           7 | Drama       | 2006-02-15 04:46:27 |
|           8 | Family      | 2006-02-15 04:46:27 |
|           9 | Foreign     | 2006-02-15 04:46:27 |
|          10 | Games       | 2006-02-15 04:46:27 |
+-------------+-------------+---------------------+

+---------+-------------+---------------------+
| film_id | category_id | last_update         |
+---------+-------------+---------------------+
|       1 |           6 | 2006-02-15 05:07:09 |
|       2 |          11 | 2006-02-15 05:07:09 |
|       3 |           6 | 2006-02-15 05:07:09 |
|       4 |          11 | 2006-02-15 05:07:09 |
|       5 |           8 | 2006-02-15 05:07:09 |
|       6 |           9 | 2006-02-15 05:07:09 |
|       7 |           5 | 2006-02-15 05:07:09 |
|       8 |          11 | 2006-02-15 05:07:09 |
|       9 |          11 | 2006-02-15 05:07:09 |
|      10 |          15 | 2006-02-15 05:07:09 |
+---------+-------------+---------------------+

Answer 1

您需要将categories与films进行左连接，以便在结果中包含所有类别，即使是那些没有电影的类别，并按类别分组进行计数：

select c.name, count(f.film_id) counter
from categories c left join films f
on f.category_id = c.category_id
group by c.category_id, c.name

请参阅演示。
结果（对于您的样本数据，其中包括表films中的category_id ，表categories中不存在，因此它们不被计算在内）：

| name        | counter |
| ----------- | ------- |
| Action      | 0       |
| Animation   | 0       |
| Children    | 0       |
| Classics    | 0       |
| Comedy      | 1       |
| Documentary | 2       |
| Drama       | 0       |
| Family      | 1       |
| Foreign     | 1       |
| Games       | 0       |