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Relational Algebra
Relational algebra is a set of algebraic operators and rules used to manipulate relational tables to generate the
required information. Relational algebra takes relations (tables) as its operands and returns relations (tables) as its
result. Relational algebra consists of eight operators:
SELECT, PROJECT, JOIN, INTERSECT, UNION, DIFFERENCE, PRODUCT, and DIVIDE.
Select Operation
Select operation is a unary operation that selects 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). For example, to select those tuples
from the table student whose Rollno is greater than 103, we write: σ Rollno > 103 (Student)
Table: Student Result of Selection
Rollno Name Rollno Name
100 Ronny 104 Kumar
101 Vivek 105 Jimmy
102 Sudha 106 John
103 Punit
104 Kumar
105 Jimmy
106 John
Another example, σ name = Kumar (Student).
Rollno Name
104 Kumar
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). Suppose, we have to select the names of those students who scored more than 250, we shall
write:
π Name (sigma marks>250 (Student))
Table: Student Result of Projection
Rollno Name Marks Name
101 Vivek 256
102 Sudha 210 Vivek
103 Punit 269 Punit
104 Kumar 287
105 Jimmy 210 Kumar
106 John 276 John
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