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## SELECT (σ) in Relational Algebra

Operation
SELECT
Type of operation
Unary
Syntax
σ <selection condition> (R)
In the above expression,
σ – selection operator
<selection condition> - <Attribute> <comparison operator> <value/other attribute>
comparison operators are {=, <, <=, >, >=, <>}
 Valid Examples Invalid regno = ‘BIT001’ age < 65 student.regno = subject_taken.regno regno = <’BIT001’> 65 = age

More than one conditions can be used and those conditions can be connected using logical connectives ^ (and), v (or), and ¬ (not).
Example:
regno = ‘BIT001’ ^ age<65
student.regno = subject_taken.regno v age > 20 ^ age<65
R – Relation name [or another expression]

Function
Selects all tuples (records) that satisfy the selection condition from a relation R (or an expression).
It filters records according to the condition. It deals with the tuples (records) of a relation.

In SQL, the selection operation is specified in WHERE clause of the query.
Example 1
RA: σage>15 (student)
SQL: SELECT * FROM student WHERE age>15;
 STUDENT Regno Name Age Phone BIT001 Ram 15 8652398452 BIT023 Selvi 17 7894562310 MIS089 Kumar 16 8562314701

Result: Finds all students with age greater than 15
 RESULT Regno Name Age Phone BIT023 Selvi 17 7894562310 MIS089 Kumar 16 8562314701
Example 2
RA: σage>15^phone=9202092020 (student)
SQL: SELECT * FROM student WHERE age>15 and phone = 9202092020;

Result: Finds all students whose age greater than 15 and having phone 9202092020.
 RESULT Regno Name Age Phone

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