Consider the following MAILORDER relational schema describing the data for a mail order company:
The attribute names are self-explanatory. "qoh" stands for quantity on hand. Specify and execute the following queries using the RA interpreter 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 only from employees living in the city of Wichita.
e. Retrieve the names of customers who have ordered all parts costing less than $20.00.
f. Retrieve the names of customers who have not placed a single order.
g. Retrieve the names of customers who have placed exactly two orders.