CS186-L16: DB Design: FDs and Normalization
Contents
Functional Dependencies
big picture
Def
- X -> Y means X determines Y, X and Y can be a single column or multiple columns
- F+ means that to be the set of all FDs that are implied by F
terminology
Anomalies
可以用FD分解relation从而避免冗余
Armstrongs Axioms
Attribute Closure
wanna check if X->Y is in F+
BCNF and other Normal Forms
Basic Normal Form
NF is a def of data model!
Boyce-Codd Normal Form
Lossless Join Decompositions
Def: decomposition won’t create new attributes, and will cover the original attributes (不是完全无重叠分割)
Problems with Decompositions
- can loss info and unable to reconstruct the original data
- do not loss data actually, in fact, we gain some dirty data
- Dependency check may require joins
- some queries may be more expensive, since join is required
Lossless Join Decompositions
定义
定理
Dependency Preservation and BCNF Decomposition
Def: Projection of set of FDs F:
Def: Dependency Preserving Decomposition
BCNF Decomposition
没有听懂
但是dependency没有保留
所以BCNF可以lossless，但是不一定保留所有的dependency