How could/should I write this SQL query?

Question

I need to write an SQL query to list a customers prices for our products. We have a standard price list, customer_no = 0 , and a customer specific price list, customer_no = XXXX .

I am having trouble understanding how I can get the query to return the customer specific price for a product, if they've been given such, or if not fall back to the standard price.

To get all products and prices on the standard price list

select     prices.product_id, products.product_desc, prices.m2
from       prices, products
where      prices.product_id = products.product_id
and        prices.customer_no = 0
order by   prices.product_id asc

To get all products and prices that the customer has been specifically quoted for

select     prices.product_id, products.product_desc, prices.m2
from       prices, products
where      prices.product_id = products.product_id
and        prices.customer_no = $_SESSION['customer']
order by   prices.product_id asc

How can I perform the first query, but if the customer has their own price then replace it with that? Is this even possible?

Thanks in advance.

Steve

Edit: Sorry, missed the third line in both queries in the original post.

Answer 1

You have to join with the prices table twice, once for the list prices and then for the quoted prices. Use a LEFT JOIN for the latter, since some products won't have a quoted price. Then use NVL to default from the quoted price to the list price.

SELECT products.product_id, products.product_desc, NVL(p2.m2, p1.m2)
FROM products
JOIN prices p1 ON p1.product_id = products.product_id
LEFT JOIN prices p2 ON p2.product_id = products.product_id 
                    AND p2.customer_no = $_SESSION['customer']
WHERE p1.customer_no = 0

Answer 2

Try:

select     prices.product_id, products.product_desc, prices.m2
from       prices, products
where      prices.customer_no = nvl($_SESSION['customer'], 0)
order by   prices.product_id asc