Question

According to my professor in Intro. to Database Theory, there are NO examples in existence to show when this occasion would arise which seems a little bizarre considering its a specific part of the theory.

What I’m looking for is just an example relation that is in 4th normal form and such that 5th normal form decomposition can be performed. OR (which is probably more likely) some insight to this (my teacher had none) which seems almost paradoxical if it is in fact impossible to create…

Answer 1

From “Introduction to Database Systems”:

Consider a table {supplier, part, project} with the meaning that the indicated supplier supplies the indicated part to the indicated project.

If there is a constraint to the effect that for all suppliers sx, all parts py and all projects jz :

• if supplier sx supplies part py, AND
• supplier sx supplies to project jz, AND
• part py is used in project jz,

then it must follow that supplier sx supplies part py to project jz.

Supposing such a constraint is in effect, then the single-table design with three columns is 4NF, but not 5NF.

The update anomalies are as follows for the three-column design are as follows:

Suppose the table has two rows, (s1,p1,j2) and (s1,p2,j1). If we insert the row (s2,p1,j1) without also inserting (s1,p1,j1), then the constraint would be violated.

Suppose the table has four rows, (s1,p1,j2), (s1,p2,j1), (s2,p1,j1) and (s1,p1,j1). If we delete the row (s1,p1,j1) without also deleting one of the three others, then once again the constraint would be violated.