Question

This one is quite tricky i’ve been scratching my head all day.

I have a table containing billing periods

ID     | DATEBEGIN | DATEEND  | CUSTOMER_ID
=======+===========+==========+=================
1      | 1/1/2011  | 30/1/2011 | 1

I have a table containing ‘sub customers’

ID     | NAME    
=======+===========
1      | JOHN
2      | JACK
3      | Elwood

I have a table containing items purchased on a subscription (wholesale account

ID     | DATE      | SUBCUSTOMER_ID | CUSTOMER ID
=======+===========+================+==============
1      | 15/1/2011 | 1              | 1
2      | 18/1/2011 | 1              | 1
3      | 25/1/2011 | 2              | 1
4      | 28/1/2011 | 3              | 1

So I want to count ‘credits’ to deduct from their account. So the subscription is per ‘sub customer’.

So at the end of the billing period (30/1/2011 from first table). I need to count the distinct sub customers (there are 3). They are charged pro-rata from the first purchase they make during the billing period.

Days Having Subscription  |  SUBCUSTOMER_ID   |  Pro Rata Rate   | CUSTOMER_ID
==========================+===================+==================+==============
3                         |  3                | 3/30             | 1
6                         |  2                | 6/30             | 1
16                        |  1                | 16/30            | 1

The output should therefore be

CUSTOMER_ID | BILLING CREDITS
============+========================
1           | 25/30

I have to count it pro rata, previously it would be unfair to bill a full period even if they purchase an item 1 day prior to the billing date

Answer 1

SELECT  customerId, SUM(DATEDIFF(dateend, fp) + 1) / (DATEDIFF(dateend, datestart) + 1)
FROM    (
        SELECT  b.*, MIN(date) AS fp
        FROM    billing b
        JOIN    purchase p
        ON      p.customerId = b.customerId
                AND p.date >= b.datebegin
                AND p.date < b.dateend + INTERVAL 1 DAY
        GROUP BY
                b.id, p.subcustomerId
        ) q
GROUP BY
        customerId