A column of a table represents an attribute. It can also be described as a
that a domain D has in arelation scheme R.
Candidate key:
Each attribute or minimal combination of attributes that uniquely
identifies any tuple in a relation is called a candidate key. Minimal means
that removing an attribute leaves the key without the ability to uniquely
identify any tuple and therefore not being a candidate key anymore.
domain:
A domain D is a set of atomic values
that defines the value range of attributes.
First normal form:
A relation is in first normal form if every attribute in every
row can contain only one single (atomic) value.
Foreign key:
An attribute in a relational scheme R1 is a foreign key if it is
in relationship with a primary key from R2 and if:
The domain (value range) of the foreign key in R1 is the same
as the domain of the primary key in R2.
The set of values of the foreign key in R1 is a subset of all
primary key values in R2.
Foreign keys usually are marked dotted
underlined.
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.
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.
Identification key:
If every attribute B of R is functionally dependent of A, than
attribute A is a primary key.
Primary key:
The primary key is one chosen key candidate that acts as the
identification key for a relation. Usually this is a short attribute like a
ID-number (identification key) or username. Attributes of the primary are
commonly underlined.
Relation:
A relation r
is one instance of the
relation scheme R(A1, A2, ..., An) containing a set of n-tuples r ={t1, t2, ...
, tn}.
Relational database scheme:
A relational database scheme is a set
of relation schemes S = {R1, ..., Rn} together with a set of integrity
conditions. A relational database is a relational database scheme together with
a database instance.
Relation scheme:
A relation scheme R
(A1, A2, ..., An)
is made up of a relation name R and a list of attributes {A1, A2, ...,
An}.
Second normal form:
A relation is in
second normal form if
it is in 1NF and every non key attribute is fully functionally dependent on the
primary key.
Third normal form:
A relation is in third normal form if
it is in 2NF and no non key attribute is transitively dependent on the primary
key.
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.
Tupel:
A tuple t is a list with n values
t = <d1, d2, ..., dn> where each value di is either an
element of the domain Di or
NULL. A tuple is a record in a relation (row in a table).