So far, I have done this 

SELECT cfirstname, clastname, annualincome
FROM creditrating
INNER JOIN customer ON creditrating.creditid = customer.creditid
INNER JOIN encounter ON customer.customerid = encounter.customerid
INNER JOIN salesperson ON encounter.salesid = salesperson.salesid
WHERE annualincome >=80000
AND sfirstname = ‘Katie’

 
but how can I make sure that the customer purchased from Katie?
how do you link PURCHASE?