doubt in databases

Question

Comments

ok okk got ur point u decompose intyo that decompositions because u want lossless dependency that is why u decompose into  BC----> D dependency because BCNF must satisfy lossless but no gurantee about dependency reserving ...

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@srestha Like @Shaik Masthan , I also took care of dependency every time. My teacher also said not to do so. 

Like @Shaik Masthan I also thought "the table R1 have the following Dependencies = { BC --> D }" but my Sir solved and and said BCD is the candidate key as there are no FD's for the table.

Could you please explain why we don't derive dependencies at every decomposition? @Arjun Sir, could you please see this too? Thanks :)

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@surbhijain93 Please add your answer. 

Answer 1

Let's First, decompose into BCNF.

Candidate keys are (AB, BC), So, prime attributes are A, B, C.

 There is a  partial dependency here C --> A.

1. After Converting 2NF we get

                               R1(AC)        ||        R2(BCDE)

                              C --> A         ||    D -- > E

Note: We can't write AB --> CDE either in R1 or R2

 

2. Here C ---> A is BCNF because It Has Only Two Attributes,  In relation, R2 D can't derive BCDE,  D is not candidate key here the divide further  

                             R1(AC)        ||        R2(DE)         || R3(BCD)

                              C --> A         ||    D -- > E           || Nothing here

Here the above relation is in BCNF. But we can't CDE from this relation. So It's not FD preserving.

Comments

@Arjun sir and @srestha Ma'am, continuing to my above comment, in the above solution 

1. After Converting 2NF we get 

                               R1(AC)        ||        R2(BCDE) 

                              C --> A         ||    D -- > E 

Why don't we also include BC->DE in R2? cause BC+=BCADE. 

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Also in this ques:

Why are we deriving B->A in table AB and C->B in BC?

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from where will you get BC->DE?

Question is unclear to me. Can u post a separate question for ur doubt?