Composite key
I design a database system with auction based. In this system, one kind of item can have multiple category and one category can possess multiple items, thus create a independent table which uses composite primary key. Notice that primary key can have multiple columns, not only 1 column. This applies to all many-to-many relationship.
Functional Dependency
Use FD & FD Closure to determine good DB and make normalization. Not always make BCNF decompotion. When you make a composition, the attributes are splitted. FD in 1 attr set only refer to all attr are contained in this datasets.BCNF decompisition will splitted into all trivial && super key datasets.
BCNF after decompotion all nontrival FD/ all FD are in super key
3NF
BCNF implies 3NF.
Relation R is in 3NF, if FD X->A, is trival FD, X is super key, A is part of some key of R.Decomposition into 3NF is always feasible. Lossless join and property preserving.