Question

Classify the following operations in relational algebra into unary and binary operations:

(1) UNION

(2) SELECT

(3) SET DIFFERENCE

(4) PROJECT

(b) Explain about SELECT, INTERSECTION and SET DIFFERENCE operations with example.

Answer 1

(a) Unary Binary

(2) SELECT

(1) UNION

(4) PROJECT

(3) SET DIFFERENCE

(b) SELECT operation

SELECT operation is used to select tuples in a relation that satisfy a selection condition. Greek letter cr (sigma) is used to denote the operation.

Syntax,

σ_Condition(relation)

eg. σ_{Salary<100000}(EMPLOYEE)-selects tuple whose salary is less than 10000 from EMPLOYEE relation.

Intersection operation 

This operation returns a relation consisting of all the tuples appearing in both of the specified relations. It is denoted by n. It can takes place only on compatible relations, e.g. FOOTBALL ∩ CRICKET returns the players who are in both football and cricket teams.

Set difference operation (-)

All tuples appearing in the first relation and not in the second