- is a
medium level model
with moderate amount of
details
on how the
data will be
stored
- In contrast: the ER-model is a high level model showing very little details on how the data will be stored
- So the Relational Model has
enough details
to describe the
files used
to
stored the
data !!!
- The Relational Model uses
only one concept
in the description:
- A relation
BUT:
- The relation concept in the Relational Model is not the relation concept used in Mathematics (which is a fixed set (collection) of elements)
|
|
- Relation:
- Relation =
a table
of values
(So this concept quite different from a relation in Mathematics)
Example: a relation
+-------+------------+---------------+----------+-----+ | 12345 | John Doe | 123 My Ln | 123-4567 | 3.5 | +-------+------------+---------------+----------+-----+ | 22222 | James Bond | 10 Downing St | 777-7777 | 3.7 | +-------+------------+---------------+----------+-----+ | 56789 | Jane Doe | 321 Your Way | 876-5678 | 3.8 | +-------+------------+---------------+----------+-----+
Comment:
- We can see the
file structure here !!!
Each row of a table is a data record in a file !!!
- Relation =
a table
of values
- Definitions (terminology):
- Relation name =
name
(identification) of the
relation (= table)
- Attribute name =
name (identification) of
a column
of the relation (= table)
- Tuple = a row of values of the relation (table)
Schematically:
- Relation name =
name
(identification) of the
relation (= table)
- More definitions:
- Domain =
set of
atomic values
that are assigned to an attribute
Note:
- The set of attribute values in a domain must be atomic (= singleton values, not composite nor multi-valued !!!)
- Relation schema consists of:
- Relation name and
- All its attribute names
Example: Relation Schema of Student
student(ID, Name, Address, Phone, GPA)
- Degree of a relation =
the
number of attributes
in the relation schema
Example:
student(ID, Name, Address, Phone, GPA) degree(Student) = 5 (there are 5 attributes)
Note:
- degree of a relation is very different from the degree of a ER-relationship
(Degree of a ER-relationship = the number of entities participating in the relationship)
- Domain =
set of
atomic values
that are assigned to an attribute
- Properties of relations (tables):
- Tuples
in a relation (= table) are
unordered
Note:
- This may not sound like a big deal to you now, but you have to
realise that before the Relational Database Model was introduced,
the existing database models (the hierarchical and the network
database models) uses
file pointers (= disk locations) to
access data in different files.
- These file pointers make it impossible to move records within a file....
- This may not sound like a big deal to you now, but you have to
realise that before the Relational Database Model was introduced,
the existing database models (the hierarchical and the network
database models) uses
file pointers (= disk locations) to
access data in different files.
- Attributes (= columns)
of a relation (= table) are
ordered
Note:
- This property makes it very difficult to add or remove attributes from/to relations (= tables) once the relations are populated with data....
- Attribute values
are
atomic
(This is a natural result of the property that the domains only have atomic values - see above).
- Tuples
in a relation (= table) are
unordered
- Notations:
- A relation schema
is denoted as
R(A1, A2, ..., An)
- A tuple
of a relation
is denoted as
t(a1, a2, ..., an)
- t[Ai]
= (denotes) the value
of the attribute
Ai
in the tuple t
( The value is equal to ai )
-
t[Ai, Aj, Ak]
= (denotes) the value
of attributes
Ai, Aj, Ak
in tuple t
( The valus is equal to (ai, aj, ak) )
- A relation schema
is denoted as
R(A1, A2, ..., An)
- Examples:
Relation Schema: Student(StuID, Name, Address, Level, GPA) Tuples: t1(1234, John, 123 South Ln, 3, 3.5) t2(2222, Jake, 456 North Ln, 2, 3.8)
t1(SSN) = 1234 t2(SSN) = 2222 t1(SSN, Name) = (1234, John) t2(SSN, Name) = (2222, Jake)