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Normalization and Normal Forms,
Example,
(more info:
I,
II,
III
)
The goal of normalization is to create a set of relational tables that
are free of redundant data and that can be consistently and correctly
modified.
Normalization theory is based on the concepts of normal forms. A
relational table is said to be a particular normal form if it
satisfied a certain set of constraints. There are (at least) five
normal forms (NFs), the first three are by E. F. Codd:
- First NF:
- A relation cannot have repeating fields or groups (no field must
have more than one value).
- Second NF:
- + every nonkey field must be functionally dependent on all of the key.
- Third NF:
- + a nonkey field cannot be functionally dependent on another nonkey field.
- Fourth NF:
- A relation cannot contain two or more independent multivalued attributes
of an entity.
- Fifth NF:
- Roughly, a relation cannot be reconstructed from several relations
each having fewer fields and not all having the same key.
In any relation, attribute A is functionally dependent on attribute B
if, regardless of any insertions or deletions, the value of B
determines the value of A; in other words, only one value of A occurs
with a particular value of B.
Commonly all tables in a relational database should
be in the third normal form (3NF).