该表是4NF吗？

Question

I'm trying to design a database. I have a table as follows:

Tag     Post     File
A        a         1
A        a         2
A        a         3
A        b         4
B        a         1
B        b         1
B        c         1
B        d         1

Each post may have more than one tag and each file may have more than one tag. Aside from the tags the posts and files are unrelated. Obviously just from looking at the table there are going to be redundancy issues for example the post-tag pair Aa is repeated 3 times unnecessarily, same with the post-file pair B-1. However I've looked at the definitions for 1NF up to 4NF and it seems to be in all of them.

What did I miss?

I thought that if a table had redundancies then it's not normalized. But it does seem to be normalized up to 4NF. So I must have gone wrong somewhere.

Answer 1

Based on your description you should break down that table into 2 tables to model the One-To-Many relationship between "Post and Tag", and "File and Tag"

POST   TAG
a       A
b       A
a       B
b       B
c       B
d       B


FILE   TAG
1       a
2       a
3       a
4       b
1       b
1       c
1       d

Take a look at the first example here:

http://en.wikipedia.org/wiki/Fourth_normal_form

It is typically the same case you are describing in your question.

Here's a useful question too:

Database Normalization