- Superkey：设 K 为 R< U , F >的属性或属性组，若 K→U，则称 K 为 R 的超码
- Candidate key：设 K 为 R< U , F >的超码，若 K→ when f U，则称 K 为 R 的候选码
- Primary key：若 R(U , F) 有多个候选码，则可以从中选定一个作为 R 的主码
Goals
Our goals of database design with functional dependencies are:
- BCNF.
- Losslessness.
- Dependency preservation
Definition:
find a = find b, ,
In general, a functional dependency of the form α → β is trivial if β ⊆ α
Trivial
if is a subset of
部分函数依赖
对子集函数依赖
Closure
We shall use the notation to denote the closure of the set F, that is, the set of all functional dependencies that can be inferred given the set F. contains all of the functional dependencies in F.