postgres

Question

我在数据库中遇到问题。例如，如果我正在构建一个博客，我博客上的帖子应该有多个标签。我想在我的帖子表格中有tag_id列，对应tag_id表格中的tags，但这意味着每个帖子只能有一个标记。我希望每个帖子都能有多个标签。如何在单行中的tag_id列中输入多个ID（对应于特定帖子）。如果它不能与ids一起使用，它是否可以与标签名称一起使用？我意识到我可以输入多个标签名称，但是如何将标签名称与tags_names表中的tags相关联？

Answer 1

参考我几年前写的文章Implementation for tag based mail system by JOINS using MySQL Database，我会说你所做的是对的。所以最终，你的表应该是这样的：

<强>帖子

mysql> SELECT * FROM `mailserver`.`mails`;  
+----+------------------+----------------------------------------------+
| ID | Subject          | Content                                      |
+----+------------------+----------------------------------------------+
|  1 | Welcome Home     | Hey man, Welcome to your new house.          |
|  2 | Hi               | Hey there, wanna see what you doing at home! |
|  3 | Your promotion   | This is to say about your promotion!         |
|  4 | What the hell?   | College is really bad!                       |
|  5 | My Project Work! | I have hereby attached my project work!      |
+----+------------------+----------------------------------------------+
5 rows in set (0.00 sec)  

<强>标签：

mysql> SELECT * FROM `mailserver`.`maillabel`;  
+----+-----------+--------+
| ID | LabelName | MailID |
+----+-----------+--------+
|  1 | inbox     |      1 |
|  2 | inbox     |      3 |
|  3 | inbox     |      5 |
|  4 | personal  |      1 |
|  5 | friends   |      2 |
|  6 | office    |      3 |
|  7 | personal  |      4 |
|  8 | college   |      5 |
+----+-----------+--------+
8 rows in set (0.02 sec)  

关系表

mysql> SELECT * FROM `mailserver`.`maillabel`;  
+----+-----------+--------+
| ID | LabelName | MailID |
+----+-----------+--------+
|  1 | inbox     |      1 |
|  2 | inbox     |      3 |
|  3 | inbox     |      5 |
|  4 | personal  |      1 |
|  5 | friends   |      2 |
|  6 | office    |      3 |
|  7 | personal  |      4 |
|  8 | college   |      5 |
+----+-----------+--------+
8 rows in set (0.02 sec)  

最终，你会对两件事感兴趣。

1. 如何从一个特定标签获取帖子？
2. 如何获取某个特定帖子的所有标签？

回答上述问题：

特定代码的所有帖子inbox：

mysql> SELECT *  
       FROM   `mailserver`.`mails`
              JOIN `mailserver`.`maillabel`
                ON `mailserver`.`mails`.`id` = `mailserver`.`maillabel`.`mailid`
       WHERE  `mailserver`.`maillabel`.`labelname` = 'inbox'; 
+----+------------------+-----------------------------------------+----+-----------+--------+
| ID | Subject          | Content                                 | ID | LabelName | MailID |
+----+------------------+-----------------------------------------+----+-----------+--------+
|  1 | Welcome Home     | Hey man, Welcome to your new house.     |  1 | inbox     |      1 |
|  3 | Your promotion   | This is to say about your promotion!    |  2 | inbox     |      3 |
|  5 | My Project Work! | I have hereby attached my project work! |  3 | inbox     |      5 |
+----+------------------+-----------------------------------------+----+-----------+--------+
3 rows in set (0.00 sec)  

回答第二个问题......

获取某个帖子的标记：

mysql> SELECT labelname  
       FROM   `mailserver`.`maillabel`
              JOIN `mailserver`.`mails`
                ON `mailserver`.`mails`.`id` = `mailserver`.`maillabel`.`mailid`
       WHERE  `mailserver`.`mails`.`id` = 3; 
+-----------+
| labelname |
+-----------+
| inbox     |
| office    |
+-----------+
2 rows in set (0.00 sec)  

此实现适用于 MySQL Server 5.x 。此概念可应用于Oracle，Microsoft SQL Server，IBM DB2等。即使我们可以将其应用于MS Access。只是每个数据库中查询的语法都不同。