Question

can any one tell what are the steps to find whether there is Dependency Preserving Decomposition or not?

Answer 1

Ok let me tell you steps to check dependency preserving and i will take an example and explain it.

first of all in the question a relation will be given along with some functional dependencies for eg R(ABCDE) where AC->B,C->D,B->E,B->CD

Now this relation is decomposed in three relations(let us assume) R1(AB),R2(BC),R3(CDE).Now for these decomposed relations we have to find functional dependencies.

For relation R1(AB): FDs for this relation can be derived by looking at original relation R(ABCD) .But in R1(AB) we have two attributes A,B so we have to consider these two attributes only.In original FD AC->B but we have only 2 attributes in R1 i.e., A,B so we have take A->B(c is ignored because C is not the attribute of R1) similarly B->E but E is not attribute of R1 so we cannot take this FD. So there would be only 1 FD for R1(AB) i.e., A->B.

Similarly FDs for R2(BC) would be B->C and for R3(CDE) C->D

now from R1,R3 and R2 we got following set of FDs:

A->B,B->C,C->D

If From these set of FDs we are able to find all FDs of original relation R(ABCDE) directly or indirectly then we can say that the decomposition is dependency preserving.

Now we can see that C->D is in the set of FDs of decomposed relation as well as in original FD.similarly B->C is also there and B->D can be derived indirectly from set of decomposed FDs i.e., B->C and C->D which means B->D...But any how we cannot derive AC->B bcoz there is not any combination of A and C in set of FDs of decomposed relation.Hence this decomposition is not preserving dependency

 

FDs for R3(BD):

Comments

I think in this statement there is error

For relation R1(AB): FDs for this relation can be derived by looking at original relation R(ABCD) .

---

You do one thing.. Just do some questions.. You will learn more

---

Have you any link where I will get those questions?

Answer 2

I am taking an example this might be dependency preserving or not. I want to explain you the concept only

Suppose there is a relation R(ABCDE) and corresponding functional dependencies

A->B

B->C

C->D

E->B

and this relation is decomposed in three relations R1(AB),R2(BC),R3(CDE)

Now you need to write Functional dependencies for each relation for example for R1(AB):

A->B; for R2(Bc): B->C;  R3:C->D; Now from these set functional dependencies of decomposed  relation if we are able to find all the functional dependencies of original Relation R(ABCDE),Whether directly or indirectly then we can say that decomposition of R(ABCDE) is dependency preserving otherwise it is not

Comments

please complete your answer.

Answer 3

Consider this example.... All the fds of original relation are recovered from fds of decomposed relations

Comments

brother I am talking about the steps and please write steps to understand it.