2. Consider the following MAILORDER relational schema describing the data for a mail order company. PARTS(Pno, Pname, Qoh, Price, Olevel) CUSTOMERS(Cno, Cname, Street, Zip, Phone) EMPLOYEES(Eno, Ename, Zip, Hdate) ZIP CODES(Zip, City) ORDERS(Ono, Cno, Eno, Received, Shipped) ODETAILS(Ono, Pno, Qty) Qoh stands for quantity on hand: the other attribute names are self-explanatory. Specify the followin queries in relational algebra on the MAILORDER database schema. a. Retrieve the names of parts that cost less than $20.00. b. Retrieve the names and cities of employees who have taken orders for parts costing more than $50.00 c. Retrieve the pairs of customer number values of customers who live in the same ZIP Code. d. Retrieve the names of customers who have ordered parts from employees living in Wichita. e. Retrieve the names of customers who have ordered parts costing less than $20.00. f. Retrieve the names of customers who have not placed an order. g. Retrieve the names of customers who have placed exactly two orders. GIRADEBOOK relational schema describing the data for a grade book