Page 131 - Touhpad Ai

Selection Operation
The selection operation is a unary operation that chooses only some of the tuples (records) of the relation. The selection
operation yields a subset of the tuples (records) from a relation (table) that satisfies a given condition. This yields a
horizontal subset. The selection is denoted by lower case Greek letter, sigma (σ).
The syntax of the Selection operation: σ condition (Relation)

For example, to select those tuples from the Student relation where the Roll_No attribute is greater than 103, you
can write:

σ (Student)
Roll_No > 103
Table: Student Result of Selection
Roll_No Student_ Name Total_Marks Roll_No Student_ Name Total_Marks
100 Ronny 450 104 Kumar 410

101 Vivek 420 105 Jimmy 455
102 Sudha 390 106 John 430
103 Punit 480
104 Kumar 410
105 Jimmy 455
106 John 430

Another example is to retrieve all tuples from the Student relation where the Name attribute is Kumar.

σ Student_Name = “Kumar” (Student)
Result of Selection

Roll_No Student_Name Total_Marks
104 Kumar 410

Projection Operation

The projection of a relation is defined as a projection of all its tuples over some set of attributes. This operation yields
a vertical subset of the relation (table). Projection also removes duplicate tuples in the resultant table. It is denoted by
the Greek letter pi(π).
The syntax of the Projection operation: π (Relation)
Attribute1, Attribute2,…
For example, we have to retrieve the student names from the Student relation where the Total_Marks attribute is
greater than 470, you can write:
π (σ (Student))
Name Total_Marks>470

Table: Student Result of Projection
Roll_No Student_Name Total_Marks Student_Name
100 Ronny 450 Punit
101 Vivek 420
102 Sudha 390
103 Punit 480
104 Kumar 410
105 Jimmy 455
106 John 430

Mathematics for AI 129