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Dependency Preserving lossless 3NF decomposition

+4 votes
As far as I remember, we did not learn an algorithm for making a lossless 3NF decomposition that is dependency preserving.

(In the presentation, it says it's "not so easy" and refer to the book, chapter 3.5; the book says it's beyond its scope).

However, in last year's exams there were questions that asked exactly that.

What should be the approach for such questions? Is it just trial and error?

asked Feb 3, 2018 by Assaf (31,090 points)

1 Answer

+1 vote
We may ask you to perform a 3nf decomposition lossless decomposition, using the BCNF one that you already have.

We basically ask here to adapt the BCNF results that you have, so it will also preserve the lost dependencies. I.e by trying to add them to other relations.
answered Feb 4, 2018 by Amit (27,910 points)
If we just take a decomposition and add columns to the relations in it, who says the new decomposition is still lossless?
so you mean that we can go from BCNF to 3NF with no lost dependencies
by just adding some of the lost dependencies in our BCNF relations ?
The thing is that you explictily know the following : (1) 3nf, (2) lossless decomposition (3) dependency preserving/lost dependency.
Therefore in such questions we require that you put it all together. The relations are small, and you can carefully elimniate decomposition candidates that do not qualify