What kind of FD or MVD exists in 5NF?

2019-05-24 04:48发布

问题:

From the "A Simple Guide to Five Normal Forms in Relational Database Theory":

Fourth and fifth normal forms both deal with combinations of multivalued facts. One difference is that the facts dealt with under fifth normal form are not independent, in the sense discussed earlier.

Following rule has been mentioned:

Suppose that a certain rule was in effect: if an agent sells a certain product, and he represents a company making that product, then he sells that product for that company.

ACP
-----------------------------
| AGENT | COMPANY | PRODUCT |
|-------+---------+---------|
| Smith | Ford    | car     | 
| Smith | Ford    | truck   | 
| Smith | GM      | car     | 
| Smith | GM      | truck   | 
| Jones | Ford    | car     | 
| Jones | Ford    | truck   | 
| Brown | Ford    | car     | 
| Brown | GM      | car     | 
| Brown | Totota  | car     | 
| Brown | Totota  | bus     | 
-----------------------------

Will the following MVDs exist in the ACP relation as cyclic MVDs?

  1. agent->->company
  2. agent->->product
  3. company->->product

So, can we say that the multi-valued facts in ACP are not independent facts because of the rule enforced in the relation and because of that there is no tuple generating dependency which exist in 4NF? I am not very clear about this.