问题
Let's say I have a simple database with tables 'posts' and 'tags'. Posts can have many tags and tags can belong to many posts.
What is the best way to structure the database? I thought of using a list/serialize:
tags
idx tag_id, str tag_name
posts
idx post_id, str title, list tag_ids
OR having another table with the associations. Problem is using this I don't even know how to structure the query to pull the associated tag names when I get a post.
posts
idx post_id, str title
post_tags
fk post_id, fk tag_id
I actually I don't like either of them. Is there a better way?
回答1:
The post_tags is the proper means of implementing a many to many relationship in the database.
The only addition I'd make to what you posted is that both columns in it should be the primary key to ensure there are no duplicates.
回答2:
You almost certainly will want to use an "intersection" or join table. This table can contain the primary keys of the posts and tags table, and (optionally) its own distinct primary key. Joining the three tables should be straightforward:
select ...
from post_tags
inner join posts on post_tags.postID = posts.postID
inner join tags on post_tags.tagID = tags.tagID
...
You can create a view that does the basic join which you can then reference in your code.
回答3:
Use the table with associations - this is called a junction table.
To get the tags for a post:
SELECT tag_name FROM tags, post_tags WHERE post_tags.tag_id = tags.tag_id AND post_tags.post_id = 12345;
回答4:
Use a join table
回答5:
I think this is a good explaination to start with: http://www.tonymarston.net/php-mysql/many-to-many.html
Otherwise you should have a look on object relational mapping - which does the complicated sql queries for you.
For example: http://www.qcodo.com/
来源：https://stackoverflow.com/questions/3152850/best-way-to-store-a-many-to-many-relationship-in-mysql