The Select Operation
- The select operation select tuples that satisfy a given predicate.
- The small Greek letter sigma(σ) is use to denote select operation
- The predicate appears as a subscript to σ
- The Comparison operator =,≠,<,>,≤,≥
- The Connectives and(∧ ), or( v), not(¬)
Examples
- To Select the tuples from employee relations those employees belong to CSE Department
- σdept_name='CSE'(employees)
- The employees whose salary greater than 12000/-
- σemp_salary>12000(employees)
- Department is CSE and Salary is greater than 12000/-
- σdept_name='CSE' ∧ emp_salary>12000(employees)
Project Operation
- It is Unary operation
- It selects certain columns from a table while discarding others.
- It removes any duplicated rows from the result relation.
- Denoted by the uppercase Greek letter pi (Π)
- The attribute list appears in the subscript to Π with argument relation in parentheses.
Examples:
- ΠId,name,dept,salary(employees)
- Selects only id, name, dept_salary Form employees relation.
- Πtitle,Author(Book)
- Display all title and author from book relation
Composition of Relation Operations
- The relational-algebra operations can be composed together into a relational-algebra expression it is just like composing arithmetic operations into athematic expression.
- Example
- Πname(σdept_name='CSE'(employees))
Rename Operation
- In Relational algebra, we can rename either the relation or the attributes or both.
- It is denoted by lowercase Greek letter rho (ρ)
- ρs(new attribute name)(R)
- ρs(R)
- ρ(new attribute name)(R)
Examples:
- ρt1(fn,ln,sal)(R)
- Rename relation R with new relation t1 with new name for the listed attributes
- ρ(fn,ln,sal)(R)
- Rename only attributes listed in the brackets.
- ρt1(R)
- Rename only relation R with new name t1.