Hi all,

Here is the introduction to a thought I had that is hopefully interesting for you too.

Database normalisation is based on certain assumptions (this list is not exhaustive):

1) Each table must have a primary key: One, two or eventually even more fields that uniquely identify a record. The smaller the number of such fields, the better.

2) Atomiticy: Each field decribes one and only one entity.

3) The order of the records is not important.

4) Entities must not be stored redundant.

One consequence is that we want to avoid two or more entities to be described by one field only. And we reserve a field for each of those entities. Example:

Employeenumber    Category
12345             1
42535             2
34545             1
34534             1

Now consider the following case:

Employeenumber    
1-12345           
2-42535             
1-34545             
1-34534             

We would critize the datamodel if we would encounter the above structure and were told that the employeenumber also incorporates the category. We would say that the atomiticy rule is broken.

In the next case the employeenumber also incorporates the category. But moreover, the category is also stores separately. In this case, we might argue that this is a clear case of redundancy. But is this really, always, the case?

Employeenumber    Category
1-12345           1
2-42535           2
1-34545           1
1-34534           1

Yesterday I would, today I no longer have that opinion. How about you?