Difference between revisions of "Normalization"
From Wiki Notes @ WuJiewen.com, by Jiewen Wu
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* Union: if X->Y and X->Z, then X->YZ. | * Union: if X->Y and X->Z, then X->YZ. | ||
* Decomposition: if X->YZ, then X->Y and X->Z. | * Decomposition: if X->YZ, then X->Y and X->Z. | ||
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| + | ===Entailment=== | ||
| + | A set of FDs X entails another set of FDs Y if X entails every FD in Y. That is, if every relation satisfies every FD in X, it must satisfy every FD in Y. | ||
| + | |||
| + | The closure of X, denoted as X<sup>+</sup> | ||
Revision as of 13:13, 5 August 2010
Functional Dependency (FD)
A FD, an integrity constraint, on a relation scheme R is a constraint X->Y, where X and Y are sets of attributes. For a pair of tuple t and s, they satisfies the above FD if they agree on all attributes in Y whenever they agree on all attributes in X.
Some properties of FDs.
- Trivial (Reflexive) FDs: X->Y if Y is a subset of X.
- Augmentation: if X->Y then XZ->YZ
- Transitivity: if X->Y and Y->Z, then X->Z.
- Union: if X->Y and X->Z, then X->YZ.
- Decomposition: if X->YZ, then X->Y and X->Z.
Entailment
A set of FDs X entails another set of FDs Y if X entails every FD in Y. That is, if every relation satisfies every FD in X, it must satisfy every FD in Y.
The closure of X, denoted as X+
