Question

Please question on DATABASE Function dependencey:

i) S(A, B, C, D) with FD’s A --> B, B --> C, and B --> D.

ii) T(A, B, C, D) with FD’s AB --> C, BC --> D, CD --> A, and AD --> B.

Answer 1

(i)

Functional dependency from A-->B means that for each values of A there will be a unique value in B. In the question we have 3 FD's so we can say that B is functionally dependent on A and C is functionally dependent on B and D is functionally dependent on B. One more important thing that you notice here that using A we can reach to each attribute. There is link from A to D directly. We can go from A to B then B to C and B to D. So the final conclusion is that A is primary key.

(ii)

In the second question AB is composite key and for unique values of AB we can find unique value in C.

Similarly BC is composite and we can get unique values in D for unique values in BC, Another FD's are CD-->A and AD-->B.

We usually avoid composite key dependency for higher degree normalization.

Normally one can Question you to normalize the tables using FD's. What is primary key? What is Super key? These are the common questions that can be asked.

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