SELECT Operation in Relational Algebra

Prerequisite – Relational Algebra 

Select operation chooses the subset of tuples from the relation that satisfies the given condition mentioned in the syntax of selection. The selection operation is also known as horizontal partitioning since it partitions the table or relation horizontally. 

Notation:

σ c(R)

where ‘c’ is the selection condition which is a boolean expression(condition), we can have a single condition like Roll= 3 or a combination of conditions like X>2 AND Y<1, and symbol ‘σ (sigma)’ is used to denote select(choose) operator,

R is a relational algebra expression, whose result is a relation. The boolean expression specified in condition ‘c’ can be written in the following form:

<attribute name> <comparison operator> <constant value> or <attribute name>

where <attribute name> is the name of an attribute of relation, <comparison operator> is any of the operator {<, >, =, <=, >=, !=} and, <constant value> is a constant value taken from the domain of the relation.  

Example-1:

σ Place = 'Mumbai' or Salary >= 1000000 (Citizen)
σ Department = 'Analytics'(σLocation = 'NewYork'(Manager))

The query above(immediate) is called nested expression, here, as usual, we evaluate the inner expression first (which results in relation say Manager1), then we calculate the outer expression on Manager1(the relation we obtained from evaluating the inner expression), which results in relation again, which is an instance of a relation we input.

Example-2:

Given a relation Student(Roll, Name, Department, Fees, Team) with the following tuples:

Select all the students of Team A :

σ Team = 'A' (Student)

Select all the students of department ECE whose fees are greater than and equal to 10000 and belong to a Team other than A.

σ σ Department = 'ECE' and Fees >= 10000(σTeam != 'A' (Student))

Important points about Select operationThe select: Select operator is Unary, which means it is applied to a single relation only. The selection operation is commutative that is,

σ c1(σ c2(R)) = σ c2(σ c1(R))

The degree (number of attributes) of the resulting relation from a Selection operation is the same as the degree of the Relation given. The cardinality (number of tuples) of the resulting relation from a Selection operation is,

0 <= σ c (R) <= |R|