“Lossless-Join Decomposition”的意思、由来-开放百科全书

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Lossless-Join Decomposition

释义

Lossless-join Decomposition
Check 1: Verify join explicitly Check 2: Via functional dependencies
Example
References

In computer science the concept of a Lossless-Join Decomposition is central in removing redundancy safely from databases while preserving the original data.^[1]

Lossless-join Decomposition

Can also be called Nonadditive.{{citation needed|date=July 2018}}

If you decompose a relation into relations you will have a Lossless-Join if a natural join of the two smaller relations yields back the original relation, i .e.;

If is split into and , for this decomposition to be lossless then at least one of the two following criteria should be met.

Check 1: Verify join explicitly

Projecting on and , and joining back, results in the relation you started with.^[2]

Check 2: Via functional dependencies

Let be a relation schema.

Let {{mvar|F}} be a set of functional dependencies on .

Let and form a decomposition of .

The decomposition is a lossless-join decomposition of if at least one of the following functional dependencies are in {{mvar|F}}⁺ (where {{mvar|F}}⁺ stands for the closure for every attribute or attribute sets in {{mvar|F}}):^[3]

Example

Let be the relation schema, with {{mvar|A}}, {{mvar|B}}, {{mvar|C}} and {{mvar|D}} attributes.
Let be the set of functional dependencies.
Decomposition into and is lossless under {{mvar|F}} because . {{mvar|A}} is a superkey in , meaning we have a functional dependency . In other words, now we have proven that .

^[4]^[5]

References

1. ^{{cite journal |last1=Pohler |first1=K |title=Lossless-Join Decomposition: applications in quantitative computing metrics |journal=International Journal of Applied Computer Science |date=2015 |volume=21 |issue=4 |pages=190-212}}
2. ^{{Cite web|title = Lossless Join Property|url = https://stackoverflow.com/questions/5771810/lossless-join-property|website =Stackoverflow.com|access-date = 2016-02-07}}
3. ^{{cite news | first= | last= | coauthors= | title=Lossless Join Decomposition | date= | publisher=Jan Chomicki | url =http://www.cse.buffalo.edu/~chomicki/560/handout-design.pdf | work =University at Buffalo | pages = | accessdate = 2012-02-08 | language = }}
4. ^{{Cite web|title = Lossless-Join Decomposition|url = http://www.cs.sfu.ca/CourseCentral/354/zaiane/material/notes/Chapter7/node7.html|website =Cs.sfu.ca|access-date = 2016-02-07}}
5. ^ {{dead link|date=January 2018}}

5 : Databases|Data modeling|Database constraints|Database normalization|Relational algebra

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