Posted by : Aron четвъртък, 21 февруари 2013 г.

Lossless-Join Decomposition



From Wikipedia, the free encyclopedia





In computer science the concept of a Lossless-Join Decomposition is central in removing redundancy safely from databases while preserving the original data.








Contents


[hide]


  • 1 Lossless-join Decomposition

  • 2 Example

  • 3 Normal Forms

  • 4 References



[edit]Lossless-join Decomposition


Can also be called Nonadditive. If you decompose a relation R into relations R_1 and R_2 you will guarantee a Lossless-Join if R_1R_2 = R.

Let R be a relation schema.

Let F be a set of functional dependencies on R.

Let R_1 and R_2 form a decomposition of R.

The decomposition is a lossless-join decomposition of R if at least one of the following functional dependencies are in F+(where F+ stands for the closure for every attribute in F):[1]

  • R_1 ∩ R_2 → R_1

  • R_1 ∩ R_2 → R_2


[edit]Example



  • Let R = (A, B, C, D) be the relation schema, with ABC and D attributes.

  • Let F = \{ A \rightarrow BC \} be the set of functional dependencies.

  • Decomposition into R_1 = (A, B, C) and R_2 = (A, D) is lossless under F because R_1 \cap R_2 = (A) and A \rightarrow BC so R_1 \cap R_2 \rightarrow R_1.


[edit]Normal Forms


Decomposition into BCNF or 3NF must always be lossless.

[edit]References




  1. ^ "Lossless Join Decomposition". University at Buffalo (Jan Chomicki). Retrieved 2012-02-08.





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