Question

我有3张桌子：

mysql> SELECT * FROM blogs;    
+----+-----------------------------+-------------+
| id | title                       | content     |
+----+-----------------------------+-------------+
|  1 | article about js & jquery   | lorem ipsum |
|  2 | article about php & mysql   | lorem ipsum |
|  3 | article about rails & mysql | lorem ipsum |
|  4 | article about rails & js    | lorem ipsum |
+----+-----------------------------+-------------+

mysql> select * FROM categories;
+----+---------------+
| id | name          |
+----+---------------+
|  1 | javascript    |  
|  2 | jquery        |
|  3 | php           |
|  4 | mysql         |
|  5 | ruby-on-rails |
+----+---------------+

mysql> SELECT * FROM blogs_categories;
+----+---------+-------------+
| id | blog_id | category_id |
+----+---------+-------------+
|  1 |       1 |           1 | 
|  2 |       1 |           2 |
|  3 |       2 |           3 |
|  4 |       2 |           4 |
|  5 |       3 |           4 |
|  6 |       3 |           5 |
|  7 |       4 |           1 |
|  8 |       4 |           5 |
+----+---------+-------------+

问题：如何按类别获取所有博客，但是为我提供与博客本身相关的所有类别？

示例：我想获取类别mysql中的所有博客。

到目前为止，我已尝试过此查询：

SELECT
  blogs.*,
  GROUP_CONCAT(categories.name SEPARATOR ",") AS categories
FROM
  blogs
INNER JOIN
  blogs_categories
  ON blogs.id = blogs_categories.blog_id
INNER JOIN
  categories
  ON categories.id = blogs_categories.category_id
WHERE
  categories.name = 'mysql'
GROUP BY
  blogs.id

但它给了我意想不到的结果：

+----+-----------------------------+-------------+------------+
| id | title                       | content     | categories |
+----+-----------------------------+-------------+------------+
|  2 | article about php & mysql   | lorem ipsum | mysql      |
|  3 | article about rails & mysql | lorem ipsum | mysql      |
+----+-----------------------------+-------------+------------+

我想要的结果是：

+----+-----------------------------+-------------+--------------------------+
| id | title                       | content     | categories               |
+----+-----------------------------+-------------+--------------------------+
|  2 | article about php & mysql   | lorem ipsum | php,mysql                |
|  3 | article about rails & mysql | lorem ipsum | mysql,ruby-on-rails      |
+----+-----------------------------+-------------+--------------------------+

我怎样才能做到这一点？是否可以在单个查询中执行此操作？

SQLFiddle

感谢您的帮助。

Answer 1

使用帖子后聚合检查，WHERE引用SELECT中的列，因此限制它们，HAVING引用GROUP BY中的值。

SELECT
  blogs.*,
  GROUP_CONCAT(categories.name ) AS categories
FROM
  blogs
INNER JOIN
  blogs_categories
  ON blogs.id = blogs_categories.blog_id
 JOIN
  categories
  ON categories.id = blogs_categories.category_id
GROUP BY
  blogs.id
HAVING
  SUM(categories.name='mysql')>0
;

http://sqlfiddle.com/#!2/b1780/17

MySQL使用串联查询多个表

1 个答案: