SQL Server Join AND QUERY Issue - sql

Question

I have a table billing_history which consists of the following fields
CREATE TABLE [dbo].[BILLING_HISTORY2](
[ID] [int] IDENTITY(1,1) NOT NULL,
[READING_MONTH_YEAR] [date] NULL,
[READING] [int] NULL,
[CONSUMER_ID] [int] NULL,
[payment_status] [bit] NOT NULL
)
http://www.sqlfiddle.com/#!3/892e0/5
The following queries return the MAX Paid And Max Unpaid Values for a Consumer
SELECT MAX(READING) AS 'MAXIMUM_PAID_READING',consumer_id from billing_history2
where payment_status=0 GROUP BY consumer_id;
SELECT MAX(READING) AS 'MAXIMUM_UNPAID_READING',consumer_id from billing_history2
where payment_status=1 GROUP BY consumer_id;
However when i join them to subtract the MAXIMUM_PAID_READING from MAXIMUM_UNPAID_READING to get the current reading, by joining the above two queries, it results in returning all those records which have a matched consumer_id. So if a consumer hasn't paid any bill yet, the id would be omitted in the PAID_READING and hence an INNER JOIN doesn't return any result. IF i use FULL OUTER JOIN, it return all the records but sets the difference to NULL.
I need to find out the current USAGE of the customer by subtracting it from the previous unpaid USAGE.
How do i go about joining these two queries in such a way that the resulting difference is found irrespective whether the person has paid a bill in the past or not.

Answer 1

Don't join them.
SELECT
MAX(CASE WHEN payment_status=0 THEN READING ELSE NULL END) AS 'MAXIMUM_PAID_READING',
MAX(CASE WHEN payment_status=1 THEN READING ELSE NULL END) AS 'MAXIMUM_UNPAID_READING',
MAX(CASE WHEN payment_status=0 THEN READING ELSE NULL END)
- MAX(CASE WHEN payment_status=1 THEN READING ELSE NULL END)
AS 'DIFF_READING',
consumer_id from billing_history2
GROUP BY consumer_id;
If you want handle NULL values from MAX function, use ISNULL function:
SELECT
ISNULL(MAX(CASE WHEN payment_status=0 THEN READING ELSE NULL END),0) AS 'MAXIMUM_PAID_READING',
ISNULL(MAX(CASE WHEN payment_status=1 THEN READING ELSE NULL END),0) AS 'MAXIMUM_UNPAID_READING',
ISNULL(MAX(CASE WHEN payment_status=0 THEN READING ELSE NULL END),0)
- ISNULL(MAX(CASE WHEN payment_status=1 THEN READING ELSE NULL END),0)
AS 'DIFF_READING',
consumer_id from billing_history2
GROUP BY consumer_id;

Answer 2

It's ok to use left outer join, but use isnull on your max value.
ex:
max(...) - isnull(max(...),0) as ...

Answer 3

In your case is necessary to use CASE expression
SELECT CONSUMER_ID,
MAX(CASE WHEN payment_status = 0 THEN READING ELSE 0 END) AS 'MAXIMUM_PAID_READING',
MAX(CASE WHEN payment_status = 1 THEN READING ELSE 0 END) AS 'MAXIMUM_UNPAID_READING',
MAX(CASE WHEN payment_status = 0 THEN READING ELSE 0 END) -
MAX(CASE WHEN payment_status = 1 THEN READING ELSE 0 END) AS diff
FROM billing_history2
GROUP BY consumer_id
Demo on SQLFiddle