How to do a complicated case statement

Question

In my windows form program a user can create an invoice for customers. In order to load the products to create the invoice, I need to make an sql stored procedure that takes the products from the products table. The problem is that there is something called
pricebooks in which a specific customer or group of customers can get a special price for a specific item and then when loading the products for the invoice if this customer has a special price it should put in the special price, if this customer belongs to
a group that has a special price it should show that price else it should show the regular price for that item from the products table. How would I do that? I tried something and for any row that it had a customer and item matching in pricebooks as well it
put that product down twice.
SELECT ITEM, DESCRIPTION, ONHAND, CASE WHEN P.CUSTOMER = @CUSTOMER THEN P.PRICE WHEN C.PRICEGROUP = P.PRICEGROUP AND P.CUSTOMER = @CUSTOMER THEN P.PRICE ELSE I.PRICE END AS PRICE, COST, RETAIL FROM INVENTORY I LEFT JOIN PRICEBOOKS P ON I.ITEM = P.ITEM LEFT
JOIN CUSTOMER C ON C.PRICEGROUP = P.PRICEGGROUP
The pricebook table can have either a customer with the item that has a special price or a pricegroup with an item that has a special price and in the customer table some customers have that pricegroup. (can be many pricegroups).
Debra has a question

Answer 1

Thank you so much for your help. I tweaked up the procedure a little and now it works for my code.
CREATE PROCEDURE PriceBookByCustomer (@CustomerId int)
--declare @CustomerId int
--set @CustomerId = 659
--look up customer's group from customers table
Declare @GroupId int
SELECT @GroupId = PRICEGROUP FROM CUSTOMER WHERE ID = @CustomerId
--get inventory, outer join to special customer pricing, outer join to special group pricing
SELECT I.ITEM, I.DESCRIPTION, I.ONHAND, COALESCE(CP.PRICE, GP.PRICE, I.PRICE) AS PRICE
, COST, RETAIL
FROM INV I
      LEFT JOIN PRICEBOOKS CP
      ON I.ITEM = CP.ITEM AND CP.Customer = @CustomerId
      LEFT JOIN PRICEBOOKS GP
      ON I.ITEM = GP.ITEM AND GP.GroupId = @GroupId
ORDER BY I.ITEM
Debra has a question