Subject Name Code Credit HoursDatabase System COMP 219 3

Chapter VII

Subject Name Code Credit HoursDatabase System COMP 219 3

Relational Model:• It uses a collection of tables to represent

both data and relationships among those data.

Basic StructureIn 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.

Subject Name Code Credit HoursDatabase 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}

Subject Name Code Credit HoursDatabase System COMP 219 3

Relational Model:

• A table of n attributes must be a subset of D1*D2*…….Dn• E.g..

Eid Ename Eaddr1

2

3

Aaa

Bbb

ccc

South street

West street

New

Employee Relation

Subject Name Code Credit HoursDatabase 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”.

Subject Name Code Credit HoursDatabase 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

Subject Name Code Credit HoursDatabase System COMP 219 3

Relation schema

• Empoyee(Employee_schema) denote that • Employee is a relation on Employee_schema

Subject Name Code Credit HoursDatabase 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.

Subject Name Code Credit HoursDatabase 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.

Subject Name Code Credit HoursDatabase 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,

Subject Name Code Credit HoursDatabase System COMP 219 3

Keys

• E.g..eid Ename eaddr dno

dnamedno dloc

Employee

pnamepno eid

Dept

Project

Subject Name Code Credit HoursDatabase 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