selection operation in relational algebra

selection operation in relational algebra, unary operations in relational model, unary selection operation in relational algebra

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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