Entity-Relationship ModelCont…..

CST203-2 Database Management SystemsLecture 5

There are 2 formal languages for relational model

Relational algebraRelational calculus

Relational Algebra

What is relational algebra?

The result?

Sequence of relational algebra

Divided into 2 groups

Special operationsSelection Projection Join Rename

Set operationsUnionIntersectionSet difference

Similar to normal algebra

Operation My HTML Symbol

Projection PROJECT

Selection SELECT

Renaming RENAME

Union UNION

Intersection INTERSECTION

SELECT operation

σ σ<Selection Condition> (R)

σ – SELECT operator Selection Condition : Boolean expression R : relation

If more selection conditions,Use ‘OR’, ‘AND’ and ‘NOT’

Horizontal partition

Select the students who has the GPA greater than 3.5

σ σGPA > 3.5 (STUDENT)Student TableNID Name StudentId ExamId GPA

+++++++++

SQL Result Relational algebra

select * from E where salary < 200 σsalary < 200(Employee)

select * from E where salary < 200 and nr >= 7 σsalary < 200 and nr >= 7(Employee)

id name salary

1 John 100

5 Sarah 300

7 Tom 100

idname

salary

1 John 100

7 Tom 100

idname

salary

7 Tom 100

σ<Condition1>(σ<Condition2>(R)) =

σ<Condition2>(σ<Condition1>(R))

σ<Cond1>(σ<Cond2>(σ<Cond3>(R))) =

σ<Cond1> AND <Cond2> AND <Cond3>(R)

Assignment

Write the relational algebra for selecting all details whose department is 4 and whose salary is greater than 30,000

Write it in another way

Name

ENo DOB Address

Sex Salary

DNo

PROJECT operationIf want to choose a subset of the columns in a

relation and discards the rest,Use Π

Π Name, GPA (Student)

Π <attribute list>(R))

Π<list1>(Π<list2>(R))) = Π<list>(R))

id name salary

1 John 100

5 Sarah 300

7 Tom 100

SQL Result Relational algebra

select salary from E

PROJECTsalary(E)

select nr, salary from E

PROJECTnr, salary(E)

salary

100

300

nr salary

1 100

5 300

7 100

Assignment

Write the relational algebra for selecting Name, Eno, and Address

Name

ENo DOB Address

Sex Salary

DNo

Sequence of operations STU_1stCLASS Πname(σGPA > 3.5

(STUDENT))

Assignment

Write the relational algebra for selecting Name, Eno, and Address of all male employees whose salary is greater than 20,000

Name

ENo DOB Address

Sex Salary

DNo

CARTESIAN PRODUCTeid

ename dept

1 Bill A

2 Sarah C

3 John A

dnr dname

A Marketing

B Sales

C LegalSQL Result Relational algebra

select * from Employee, Department

E X D

enr

ename dept dnr dname

1 Bill A A Marketing

1 Bill A B Sales

1 Bill A C Legal

2 Sarah C A Marketing

2 Sarah C B Sales

2 Sarah C C Legal

3 John A A Marketing

3 John A B Sales

3 John A C Legal

INNER JOIN

SQL Result Relational algebra

select * from E, D where dept = dnr

SELECTdept = dnr (E X D)

or, using the equivalent join operation

E JOINdept = dnr D

eid

name

dept

dnr dname

1 Bill A A Marketing

2 Sarah C C Legal

3 John A A Marketing

eidename dept

1 Bill A

2 Sarah C

3 John A

dnr dname

A Marketing

B Sales

C Legal

UNION operationResult 1 Πname(σGPA > 3.5 (STUDENT))

Result 2 Πname(σ(GPA > 2.5 AND GPA < 3.5)

(STUDENT))

Result Result1 υ Result 2

Result 1

Result 2

Result

Name

Amal

Sunil

Name

Kamal

Name

Amal

Sunil

Kamal

INTERSECTION operationResult 1 Πname(σGPA > 3.5 (STUDENT))

Result 2 Πname(σ(GPA > 2.5) (STUDENT))

Result Result1 Result 2

υ

Result 1

Result 2

Result

Name

Amal

Sunil

Name

Amal

Name

Amal

SET DIFFERENCEAlso called as MINUS

Result Result1 – Result2

Result 1

Result 2

Result

Name

Amal

Sunil

Name

Amal

Name

Sunil