1. Entity-Relationship Model Cont…..
CST203-2 Database Management Systems
Lecture 5

2. There are 2 formal languages for relational model
Relational algebra
Relational calculus

3. Relational Algebra

4. What is relational algebra?
The result?
Sequence of relational algebra
Divided into 2 groups

5. Special operations Set operations Selection Projection Join Rename
Union
Intersection
Set difference

6. Similar to normal algebra
Operation
My HTML
Symbol
Projection
PROJECT
Selection
SELECT
Renaming
RENAME
Union
UNION
Intersection
INTERSECTION

7. σ σ<Selection Condition> (R)
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’

8. Select the students who has the GPA greater than 3.5
Horizontal partition
Select the students who has the GPA greater than 3.5
σ σGPA > 3.5 (STUDENT)
Student Table
NID
Name
StudentId
ExamId
GPA

9. σsalary < 200(Employee) σsalary < 200 and nr >= 7(Employee) id
name
salary 1
John
100 5
Sarah
300 7
Tom
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 7
Tom id
name
salary 7
Tom
100

10. σ<Condition1>(σ<Condition2>(R)) = σ<Condition2>(σ<Condition1>(R)) σ<Cond1>(σ<Cond2>(σ<Cond3>(R))) = σ<Cond1> AND <Cond2> AND <Cond3>(R)

11. Assignment
Name
ENo
DOB
Address
Sex
Salary
DNo
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

12. PROJECT operation
If 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))

13. id
name
salary 1
John
100 5
Sarah
300 7
Tom
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

14. Assignment
Name
ENo
DOB
Address
Sex
Salary
DNo
Write the relational algebra for selecting Name, Eno, and Address

15. Sequence of operations
STU_1stCLASS Πname(σGPA > 3.5 (STUDENT))

16. Assignment
Name
ENo
DOB
Address
Sex
Salary
DNo
Write the relational algebra for selecting Name, Eno, and Address of all male employees whose salary is greater than 20,000

17. CARTESIAN PRODUCT eid ename dept 1 Bill A 2 Sarah C 3 John dnr dname A
Marketing B
Sales C
Legal
SQL
Result
Relational algebra
select * from Employee, Department
E X D
enr
ename
dept
dnr
dname 1
Bill A
Marketing B
Sales C
Legal 2
Sarah 3
John

18. INNER JOIN eid ename dept 1 Bill A 2 Sarah C 3 John dnr dname A
Marketing B
Sales C
Legal
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
Marketing 2
Sarah C
Legal 3
John

19. UNION operation Result Result1 υ Result 2
Result Πname(σGPA > 3.5 (STUDENT))
Result Πname(σ(GPA > 2.5 AND GPA < 3.5) (STUDENT))
Result Result1 υ Result 2

20. Result 1 Result 2 Result Name Amal Sunil Name Kamal Name Amal Sunil

22. INTERSECTION operation
Result Πname(σGPA > 3.5 (STUDENT))
Result Πname(σ(GPA > 2.5) (STUDENT))
Result Result Result 2 υ

23. Result 1
Result 2
Result
Name
Amal
Sunil
Name
Amal
Name
Amal

25. SET DIFFERENCE
Also called as MINUS
Result Result1 – Result2

26. Name
Amal
Sunil
Result 1
Result 2
Result
Name
Amal
Name
Sunil