单个列中的单个行中有多个条目postgres

Question

我在我的数据库中遇到问题。例如，如果我正在构建博客，我博客上的帖子应该有多个标签。我想我的帖子表中有tag_id列，与我的tags表中的tag_id对应，但这意味着每个帖子只能有一个标签。我希望每个帖子都能够拥有多个标签。如何在单行中输入我的tag_id列中的多个ID（对应于特定帖子）。如果它不适用于ID，它可以使用标签名称吗？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表中的tags_names连接？

Answer 1

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

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) 

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，你会对两件事情感兴趣。

1. 如何从一个特定的标签获取帖子？
2. 如何获得一个特定帖子的所有标签？

为了回答上述疑问：

所有帖子特定标签，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) 

并回答第二个问题...

获取一个特定帖子的标签：

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) 

该实现适用于MySQL服务器5.x。这个概念可以应用于Oracle，Microsoft SQL Server，IBM DB2等。即使我们可以将其应用于MS Access。只是查询的语法在每个数据库中都不相同。