Relational Algebra invented from first-order logic (Predicate Logic) that helps to define one or more Relations and operate through Relational Algebra Operators to produce new Relation is called "Closure".
Relational Algebra Operation can be categories into "Unary Operation", "Binary Operation" and "SET Theory Operation". Relational Algebra Operation extended with Outer Join, Aggregate Function and Generalization Projection.
Relational Algebra is a Mathematical Procedural Method that helps to define and retrieve SET of Information in the Relational Database Model. Relational Database Query Engine is internally translates SQL Query into Relational Algebra Formula that helps to Create, Rewrite, Optimize Query and Execute Database Operation.