计算特定月份mysql的平均支出（支出/人）

Question

I have three tables in my database with schema:

items (itemID: integer, description: string, price: integer)
orders (orderID: integer, itemID: integer, aID: integer, customerID: integer, date: date)
customers (customerID: integer, fName: string, lName: string)

I need to calculate the average amount of money spent by ALL customers in a given month (between 2013-03-01 and 2013-03-31) which is total amount spent/total no of customers, to do this I need to calculate the sum of the prices of all products purchased between those dates (using information from itemID in orders and price in items), I did this with this code:

SELECT SUM(price) FROM items WHERE itemID IN( SELECT itemID FROM orders WHERE date BETWEEN '2013-03-01' AND '2013-03-31' )

But i am having trouble figuring out how to divide this number by total number of customers (which can be found by counting the number of customers on the customers table - the unique key is the customerID)

Can anyone help? thanks!

Answer 1

You can use a subquery :

SELECT SUM(i.price)/(SELECT COUNT(*) FROM customers)
FROM items i
LEFT JOIN orders o ON i.itemID = o.itemID
                  AND o.date BETWEEN '2013-03-01' AND '2013-03-31'
WHERE o.itemID IS NOT NULL