Relational Algebra Operations Continued

The Division Operation

The division operation in relational algebra, denoted by ÷, is suited to queries that include the phrase “for all”.

Suppose that we wish to find all customers who have an account at all branches located in Dhaka.

Firstly we can obtain all branches in Dhaka by the expression:

We can find all (customer_name, branch_name) for which the customer has an account at a branch by writing:

 

We need to find customers who appear in r2 with every branch name in r1. The operation that provides exactly those customers is the divide operation that is, the customers who have an account for all branches in Dhaka city.