1. Subject Name Code Credit Hours
Database System COMP 219 3
Chapter VII
2. Subject Name Code Credit Hours
Database System COMP 219 3
Relational Model:
• It uses a collection of tables to represent
both data and relationships among those
data.
Basic Structure
In this model each table is called as
relation. Each relation has a no. of
row is called tuples and columns
called as fields or attributes.
3. Subject Name Code Credit Hours
Database System COMP 219 3
Relational Model:
• For an attribute, a range of permitted values
can be specified. That range is called as
Domain of that attribute.
• E.g.. Age – attribute
• Domain (Age) = {1,2…100}
4. Subject Name Code Credit Hours
Database System COMP 219 3
Relational Model:
• A table of n attributes must be a subset of
D1*D2*…….Dn
• E.g..
Eid Ename Eaddr
1
2
3
Aaa
Bbb
ccc
South street
West street
New
Employee Relation
5. Subject Name Code Credit Hours
Database System COMP 219 3
Relational Model:
• To denote the value of attribute ‘a’ on second
tuple, then it can be denoted as t2[a]=value.
• E.g..
• t2[ename]=“bbb”.
6. Subject Name Code Credit Hours
Database System COMP 219 3
Relation schema
• It is the description about the structure of the
relation and the relation instance is the
snapshot of data in the relation at a given
instant in time.
• E.g.. Employee_schema=(eid,ename,eaddr)
Name of the schema
7. Subject Name Code Credit Hours
Database System COMP 219 3
Relation schema
• Empoyee(Employee_schema) denote that
• Employee is a relation on Employee_schema
8. Subject Name Code Credit Hours
Database System COMP 219 3
Keys
• Strong Entity set:
• The Primary key of the entity set becomes the primary
key of the relation.
• Weak Entity Set:
• The table corresponding a weak entity set includes
• the attributes of the weak entity set &
• primary key of the strong entity set on which the weak
entity set depends.
9. Subject Name Code Credit Hours
Database System COMP 219 3
Keys
Relationship Set:
The union of the primary key of the related entity sets
becomes a super key of the relation.
Combined Tables:
If there exists many to one relationship set from A to B,
then the prim.key of A becomes the prim.key of the
relation & the relation consists of the attribues of A and
attributes of the relationship set.
10. Subject Name Code Credit Hours
Database System COMP 219 3
Keys
Foreign Key:
An attribute of a relation r1 that references
relation r2 is called as foreign key attribute.
Schema Diagram:
A relation schema can be depicted pictorially
by schema diagram,
12. Subject Name Code Credit Hours
Database System COMP 219 3
Query Languages:
• It is a lang in which a user requests
information from db.
Query Languages
Procedural Language Non Procedural Language
Specifies what data are required &
specify how to get those data e.g
Relational Algebra
Specifies what data are required without
specify how to get those data e.g.
Relational calculus
