Dependencies

In order to be able to normalise a relation according to the three normal forms, we must first understand the concept of dependency between attributes within a relation.

Functional dependency:

If A and B are attributes of relation R, B is functionally dependenton A (denoted A --> B), if each value of A in R is associated with exactly one value of B in R.

Example:

ID	Name
S1	Meier
S2	Weber

The attribute Name is functionally dependent of attribute ID (ID --> Name).

Identification key:

If every attribute B of R is functionally dependent of A, than attribute A is a primary key.

Beispiel:

ID	Name	Surname
S1	Meier	Hans
S2	Weber	Ueli

Attribute ID is the identification key

Full functional dependency:

We talk about full functional dependency if attribute B is functional dependent on A, if A is a composite primary key and B is not already functional dependent on parts of A.

Beispiel:

IDStudent	Name	IDProfessor	Grade
S1	Meier	P2	5
S2	Weber	P1	6

The attribute Grade is fully functional dependent on the attributes IDStudent and IDProfessor.

Transitive dependency:

If A determines B and B determines C then C is determined by (dependent on) A. We write A --> B and B --> C but not B --> A.

Example:

ID	Name	Konto_Nr	Bank_Code_No	Bank
L1	Meier	1234-5	836	UBS
L2	Weber	5432-1	835	CS

There is a transitive dependency between Bank_Code_No and Bank because Bank_Code_No is not the primary key of the relation.