MySQL: Auto incrementing combined keys

Question

Ok, I have a table that will look something like this:

Post
 ˪ Id
 ˪ Version
 ˪ Title
 ˪ Content

The idea is that the Id and Version together will be the primary key since you can have a single post multiple times but of different versions.

My question is this: I would like to have the Id to auto increment. Is this possible in a setup like this? Will things be messed up when I insert the second version of a post for example?

---

Seems like I need to clear some things up here. What I would like is to do something like this:

INSERT INTO posts VALUES (NULL, 0, "A title", "Some content");

That would create a new post with some auto incremented id. Let's say it got the id 7. I now want to create a new version:

INSERT INTO posts VALUES (7, 1, "A new title", "Some adjusted content");

Would that work? or would the id still get an auto incremented value? And even if it did work, am I doing something I shouldn't from a database design point of view, et cetera?

Answer 1

Consider the following example data:

id  version  title  content   
1     1        t1     c1
2     1        t2     c2

Now a user changes title and content of a post, creating a new row with auto-incremented id:

3     2        t3     t3

This is version two of some post, but you cannot see if it is a new version of post one (entry with id 1) or post two (entry with id 2).

You need something to identify the post. You can add a post_id column to your table as Tobias suggests in his answer.

Further, it is possible to remove the auto-incremented id completely and use a composite primary key (post_id,version) - the main point is to have a post_id which is not incremented automatically so that you can have data like that:

post_id  version  title  content   
1           1        t1     c1
2           1        t2     c2 
1           2        t3     t3

Here, it is clear that the third row represents a new version of post 1.

Answer 2

I would do it like this:

Post:  
- ID (your primary key with auto_increment)
- post_id (this is which post you mean)
- version
- title
- content

The new ID only specifies a unique ID for this dataset. The post_id then specifies to which post this dataset belongs an the version stands for the revision of the entry.

Answer 3

Why don't you use the Id as some kind of version indicator, otherwise you'd have two columns serving the same purpose which is considered a flaw in DB design.

Answer 4

You would use a composite primary key where id and version together identify the entry. You can then auto_increment id. When you add a new entry it will auto increment. When you revise a post you explicitly define id and revision and id will not be incremented.