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The SELECT operation (denoted byσ
(sigma)) is used to select a subset of the tuples from a relation based on a
selection condition. The selection condition acts as a filterand keeps only
those tuples that satisfy the qualifying condition. Tuples satisfying the condition
are selected whereas the other tuples are discarded (filtered out)
Database State for COMPANY
• Examples:
– Select the EMPLOYEE tuples
whose department number is 4:
σ DNO = 4 (EMPLOYEE)
σ Select the employee tuples
whose salary is greater than $30,000:
_SALARY > 30,000 (EMPLOYEE)
– In general, the select operation
is denoted by σ <selection condition>(R) where
the symbol σ (sigma) is used to
denote the select operator
the selection condition is a
Boolean (conditional) expression specified
on the attributes of relation R
tuples that make the condition true
are selected
(appear in the result of the
operation)
tuples that make the condition false
are filtered out
(discarded from the result of the
operation)
The Boolean expression specified
in <selection condition> is made up of a number of clauses of the form:
<attribute name>
<comparison op> <constant value>
or
<attribute name>
<comparison op> <attribute name>
Where <attribute name> is
the name of an attribute of R, <comparison op> id normally
one of the operations
{=,>,>=,<,<=,!=}
Clauses can be arbitrarily
connected by the Boolean operators and, or and not
• For example, To select
the tuples for all employees who either work in
department 4 and make over $25000
per year, or work in department 5 and make
over $30000, the select operation
should be:
_ (DNO=4 AND Salary>25000 )
OR (DNO=5 AND Salary>30000 ) (EMPLOYEE)
The following
query results refer to this database
Examples of
applying SELECT and PROJECT operations
SELECT Operation
Properties
– SELECT s is commutative:
σ
<condition1>(σ <
condition2>(R)) = σ <condition2>
(σ < condition1>
(R))
– A cascade of SELECT
operations may be replaced by a
single selection with a conjunction of
all the conditions:
σ
<cond1>( σ <
cond2> (σ <cond3>(R))
= σ
<cond1> AND < cond2> AND < cond3>(R)
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