Did I answer this correctly?

Question

yesterday I took a database exam and the question about normalization was strange. We had table R(ABCDEFG) and functional dependencies G->B, C->DG, CF->E, F-A. Which are the candidate keys for R? I only found one: CF. Then R1(DFG), which are the candidate keys for R1? I only found one: DFG. State a correct 3NF normalization for R. I stated ((C,F), E), ((G, B)), ((F), A), ((C), D)

and then the functional dependency GDF->C was added. What is now a correct 3NF normalization of R? I said ((G, D, F, C)), ((G), B), ((F, ); A), ((C), D), ((C, F), E)

Did I solve it correct?

Then even more strange, we should state what is what when the following are listed:

• Product ID
• Order number
• Customer ID
• Quantity
• Customer name
• Product name
• Date

I concluded

G= Product ID
C= Order number
F= Customer ID
D= Quantity
A= Customer name
B= Product name
E= Date

Is this correct? What does the FD GDF->C mean in plain English?

Answer 1

"Yesterday I took a database exam and the question about normalization was strange. We had table R(ABCDEFG) and functional dependencies G->B, C->DG, CF->E, F-A. Which are the candidate keys for R? I only found one: CF.

That seems OK.

Then R1(DFG), which are the candidate keys for R1? I only found one: DFG.

With the very same set of FD's ??? With no FD's at all ??? Anyway, this one seems correct too.

State a correct 3NF normalization for R. I stated ((C,F), E), ((G, B)), ((F), A), ((C), D)

((G), B) instead of ((G, B)) would be more like it.

((C), DG) instead of ((C), D) would be more like it too.

and then the functional dependency GDF->C was added. What is now a correct 3NF normalization of R? I said ((G, D, F, C)), ((G), B), ((F, ); A), ((C), D), ((C, F), E)

Addition of this FD (/constraint) doesn't alter the 3NF form. All dependencies that are expressible in the 3NF design are still "out of complete keys". The fact that this additional dependency could not be preserved by the decomposition, does not lower the normal form. It's a dependency preservation issue, not a normal form issue.

Did I solve it correct?

Best option is to ask the teachers.

"Then even more strange, we should state what is what when the following are listed:"

The folly. The question itself forces you to make assumptions. Date. What date is that ? Date of birth of the customer placing the order ? Date when the ordered product was assigned its current name ? Or perhaps Date when the order was placed ? Presumably so, but the thing is, this should be clearly spelled out in the specs and database designers should really be taught NEVER TO ASSUME ANYTHING ABOUT THE SPECS. Assumption is the mother of all screwups.

What does the FD GDF->C mean in plain English?

In plain English, and assuming your answer, it means that once a certain combination of {customer id, product id and quantity} has been used in an order, there can no longer appear a second order (with a different order id) with the very same {customer id, product id and quantity}. Or, iow : each customer can order a certain specific quantity of a certain specific product only once.