Question

Lets take stackoverflow as an example:

Post

Postid          Title          Mess
--------------------------------------------------
1               Title1         This is a question
2               Title2         This is a question1
3               Title3         This is a question2
4               Title4         This is a question3
5               Title5         This is a question4
6               Title6         This is a question5

Tags

TagId     PostId     Name
-----------------------------
1         1              Tag1
2         1              Tag2
3         1              Tag3
4         1              Tag4
5         2              Tag5
6         3              Tag6
7         4              Tag7
8         5              Tag8
9         6              Tag9
10        3              Tag10

In this design how would I get all the questions with its associated tags in one query. is this even possible?

*Edit*

select t.*, p.* from Tags t
join post p on t.postid=p.postid

Something like this would give me all the tags for each question. But I don’t think this is efficient. What do you think.

Answer 1

I think that your example is very weak. You shouldn’t have the tag name in the table you use to join posts and tags.

You should have the following tables:

• Posts: PostId, Title, Mess
• Tags: TagId, Name
• Posts_Tags: PostId, TagId

If you don’t do this then you’ll have many equal tag names when 2 posts have the same tag, do you see?

Now, in order to query these tables and get all questions with the associated tags you should do the following:

select * from posts p
join posts_tags pt on p.postId = pt.postId
join tags on pt.tagId = t.tagId

You can say it is less efficient but you’re not considering data duplication in your example, which may cause serious errors.