Question

I have this alogirthm problem, in terms of creating mysql tables, let me explain the scenario:

I have a a posts table:

Posts{id, user_id,image,tag,date}

each post can only have on tag! ONLY

Do I need to have another table to store tags! I can understand if each post has more than one tag, you’d have to create another table for it to store the tags! but with this its only one!

i.e.

If I wanted to get all the posts with tag ‘DOG’ I would use:

select all posts where tag=DOG

do you think this is wrong or bad? I’m so confused!

Answer 1

If posts in your system will really only ever need to have one tag, then technically you can just hold the tag on the Post record. You shouldn’t, though: For example, what happens when you have 250000 posts about dogs, and decide to rename the “Dog” tag to “Canine”?

A better idea is to create a Tag table and put a foreign key to it on the Post table. For example:

Post{post_id, user_id, image, tag_id, date}
Tag{tag_id, description}

But just in case you’re really faced, someday, with a scenario in which a Post can have multiple Tags…

Then you’re describing a model in which Posts and Tags exist in what’s called a Many-to-Many relationship: a given tag can be associated with any number of posts, and a given post can be associated with any number of tags.

Now you not only need a separate table for Tags, but a relationship table in between that relates posts to tags. For example:

Post{post_id, user_id, image, date}
Tag{tag_id, description}

Post_Tag{post_id, tag_id}

You would then select all posts with the “Dog” tag like so:

SELECT p.*
FROM Post p
JOIN Post_Tag pt ON pt.post_id = p.post_id
JOIN Tag t ON pt.tag_id = t.tag_id
WHERE t.description = 'Dog'