--BOOK_LIBRARY
--1. return the number of distinct customer categories

--2. retunr the number of customers

--3. return the number of student customers

--4. return the smallest, largest price

--5. return tha average book price

--6. return the number of customers for each category

--7. return the number of customers for each category, 
--   but only if the category has more than 10 customers