Example Queries on Relational Algebra

Given below are a few examples of a database and a few queries based on that.

(1). Suppose there is a banking database which comprises following tables :

Customer(Cust_name, Cust_street, Cust_city)

Branch(Branch_name, Branch_city, Assets)

Account (Branch_name, Account_number, Balance)

Loan(Branch_name, Loan_number, Amount)

Depositor(Cust_name, Account_number)

Borrower(Cust_name, Loan_number)  

Query : Find the names of all the customers who have taken a loan from the bank and also have an account at the bank.

Solution:

Step 1 :  Identify the relations that would be required to frame the resultant query. 

First half of the query(i.e. names of customers who have taken loan) indicates “borrowers” information.

So Relation 1 —–> Borrower.

Second half of the query needs Customer Name and Account number which can be obtained from Depositor relation.

Hence, Relation 2——> Depositor.

Step 2 :  Identify the columns which you require from the relations obtained in Step 1.

Column 1 :  Cust_name from Borrower 

Column 2 :  Cust_name from Depositor

Step 3 :  Identify the operator to be used. We need to find out the names of customers who are present in both Borrower table and Depositor table.

Hence, operator to be used—-> Intersection.

Final Query will be