限制基于数据库值返回的结果

Question

我有两个 table，类别和故事。

故事 table 包含按类别组织的内容。

categories_id, category_name, category_story_count
1, news, 2
2, funnies, 3

stories_id, categories_id, story_name, story_content, story_active
1, 1, "Tax Hike", "blah blah", 1
2, 1, "Tax Cuts", "blah blah", 1
2, 1, "Election", "blah blah", 1 
4, 2, "Peanuts", "blah blah", 1
5, 2, "Garfield", "blah blah", 1
6, 2, "Archie", "blah blah", 1

我想要一个查询，它将 return 基于 category_story_count 每个类别的正确故事数，如果故事是活跃的 (story_active = 1)

所以结果应该是这样的：

"news", "Tax Hike"
"news", "Tax Cuts"
"funnies", "Peanuts"
"funnies", "Garfield"
"funnies", "Archie"

两个“新闻”故事，因为新闻类别 1，有一个 category_story_count = 2，三个“搞笑”故事，因为搞笑 2，有一个 category_story_count = 3

我尝试过内部连接、嵌套和限制，但无法达到 return 我的目标。

如有任何帮助，我们将不胜感激。

编辑： MySQL版本（） 8.0.23

Answer 1

这是一个使用 window 函数的解决方案：

with cte as ( 
  select *, row_number() over (partition by c.categories_id order by s.stories_id) as rownum 
  from Categories as c join Stories as s using (categories_id) 
) select * from cte where rownum <= category_story_count;

+---------------+---------------+----------------------+------------+------------+---------------+--------------+--------+
| categories_id | category_name | category_story_count | stories_id | story_name | story_content | story_active | rownum |
+---------------+---------------+----------------------+------------+------------+---------------+--------------+--------+
|             1 | new           |                    2 |          1 | Tax Hike   | blah blah     |            1 |      1 |
|             1 | new           |                    2 |          2 | Tax Cuts   | blah blah     |            1 |      2 |
|             2 | funnies       |                    3 |          4 | Peanuts    | blah blah     |            1 |      1 |
|             2 | funnies       |                    3 |          5 | Garfield   | blah blah     |            1 |      2 |
|             2 | funnies       |                    3 |          6 | Archie     | blah blah     |            1 |      3 |
+---------------+---------------+----------------------+------------+------------+---------------+--------------+--------+

在 MySQL 8.0.23.

上测试

限制基于数据库值返回的结果

LIMIT RESULTS RETURNED BASED ON DB VALUE

mysql

greatest-n-per-group