SQL - Average number of records within a time period - sql

Question

I'm trying to compile some lifetime value information for customers within one of our databases.
We have an MS SQL Server database which stores all of our customer/transactional information.
My issue is that I don't have much experience when it comes to MS SQL Server (or SQL in general) - I'd like to be able to run a query against the database that pulls AVG number of loans, and AVG revenue based on three criteria:
1.) Loans be counted if they are 'approved'
2.) Loans from a customer_id only be counted if the first loan (first identified by date_created field) be on or after a certain 'mm/yyyy'
3.) I'm able to specify for how many months after the first 'mm/yyyy' to tally the number of loans / revenue to be included within the AVG
Here is what the database would look like:
customer_id | loan_status | date_created | revenue
111 | 'approved' | 2010-06-20 17:17:09 | 100.00
222 | 'approved' | 2010-06-21 09:54:43 | 255.12
333 | 'denied' | 2011-06-21 12:47:30 | NULL
333 | 'approved' | 2011-06-21 12:47:20 | 56.87
222 | 'denied' | 2011-06-21 09:54:48 | NULL
222 | 'approved' | 2011-06-21 09:54:18 | 50.00
111 | 'approved' | 2011-06-20 17:17:23 | 100.00
... loads' of records ...
555 | 'approved' | 2012-01-02 09:08:42 | 24.70
111 | 'denied' | 2012-01-05 02:10:36 | NULL
666 | 'denied' | 2012-02-05 03:31:16 | NULL
555 | 'approved' | 2012-02-17 09:32:26 | 197.10
777 | 'approved' | 2012-04-03 18:28:45 | 300.50
777 | 'approved' | 2012-06-28 02:42:01 | 201.80
555 | 'approved' | 2012-06-21 22:16:59 | 10.00
666 | 'approved' | 2012-09-30 01:17:20 | 50.00
If I wanted to find the avg transaction count (approved transactions), and average revenue per approved transaction for all customer's who's first loan was in/after 2012-01, and for a period of 4 months after then, how would I go about querying the database?
Any help is greatly appreciated.

Answer 1

something like this (there maybe a few typos here and there)...
you could first calculate the minimum loan date:
select customer_id, min(date_created) from table t where loan_status = 'approved' group by customer_id
then you can join to it:
select customer_id, count(date_created), avg(revenue) from table t
join (
select customer_id, min(date_created) as min_date from table t where loan_status = 'approved' group by customer_id ) s
on t.customer_id = s.customer_id
where t.date_created between s.min_date and DATEADD(month, 4, s.min_date) and t.loan_status = 'approved'

Answer 2

Rename tbl to your table name.
Specify dates in the format YYYYMMDD.
select customer_id, AVG(revenue) average_revenue
from
(
select customer_id
from tbl
group by customer_id
having min(date_created) >= '20120101'
) fl
join tbl t on t.customer_id = fl.customer_id
where t.loan_status = 'approved'
and date_created < '20120501' -- NOT including May the first, so Jan through Apr (4 months)
If you mean 4 months after each customer's first loan, leave me a comment, state whether it's 4 calendar months (e.g. 15-Jan to 15-May) or up to the last day of the 4th month (15-Jan to 30-Apr), and I'll update the answer.