Design by decomposition
Database Normalization
- "Mega" relations + properties of the data
- System decomposes based on properties
- Final set of relations satisfies normal form
- No anomalies, no lost information
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Functional dependencies => BCNF
function dependency.jpg -
Multivalued Dependency => 4NF
multivalued dependency.jpg
Relational Design.jpg
另外 Decomposition 必须满足 loseless join
loseless join.jpg
Functional Dependency
Functional Dependency 非常通用，随便举几例
- Relational design by decomposition （BCNF）
- 数据存储时压缩
- 查询时优化
key：能决定一个 relation 所有元素的元素集合
closure：给定的元素集合所能决定的所有元素
由 closure 求得 key：
考虑 relation 的所有集合（由集合大小从小到大考虑），利用 colsure 找到那些能决定 relation 所有元素的集合
BCNF
BCNF.jpg
BCNF Example1.jpg
BCNF Example2.jpg
BCNF Decomposition.jpg
BCNF Decomposition Example.jpg
Multivalued Dependency
Multivalued Dependency.jpg
Multivalued Dependency Example1.jpg
Multivalued Dependenc Example2.jpg
4NF Decomposition
4NF Decomposition.jpg
4NF Decomposition Example1.jpg
4NF Decomposition Example2.jpg
Denormalization
当然不是分解的越小越好，也要看情况。。。感觉这就是个经验问题。
Denormalization.jpg
