project operation in relational algebra

Tuesday, August 15, 2017

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

Operation

PROJECT

Type of operation

Unary

Syntax

Π _{<attribute list>} (R)

In the above expression,

<attribute list> is list of attributes separated by comma. It can also be written as Π _{attr1, attr2, …, attrn} (R).

Function

Produces a new relation with only some of the attributes of R as given in the attribute list, and removes duplicate tuples (records).

It deals with the structure of a relation (table).

In SQL, the projection operation is specified in the SELECT clause of the query.

Example 1

RA: Π _{regno, name, age} (student)

SQL: SELECT regno, name, dob FROM student;

Result: Produces a temporary relation with attributes regno, name and dob of student relation with all records.

Example 2

RA: Π _{regno,
name, age} (σ_{age>15^phone=9202092020} (student))

SQL: SELECT regno, name, age FROM student WHERE age>15 and phone = 9202092020;

Result: Produces register number, name and age of all students whose age is greater than 15 and having phone 9202092020.