finding average earning over period of employment

It has been a while since I have had to write a really advanced query and I was wondering if I could get a little bit of help from someone

find the average monthly earning while you have been an affiliate

affiliate_id, transaction_dt, earnings are the only fields that you have to worry about. I can obviously find the average earnings for a period of a year but, I would like something like

spavgmonthlyearning affiliate_id

any help would be greatly appreciate even a point in the right direction would be fantastic.so this will do it.
is this pure sql or will it only run on sqlserver
thank you for your help

DECLARE @.maxdate as datetime
DECLARE @.mindate as datetime
set @.maxdate = (select max(TRANS_DT) from dbo.affiliate_log)
set @.mindate = (select min(trans_dt) from dbo.affiliate_log)
select sum(earning)/datediff(M,@.mindate,@.maxdate)
from dbo.affiliate_log where affiliate_id = 1|||select DATEPART(m,date_field), avg(sales)
from table
group by DATEPART(m, date_field)|||It depends on what you mean by "average".

The simple average monthly earning would just be the total earnings divided by the total months.

select affiliate_id,
sum(earnings)/datediff(month, min(transaction_dt), max(transaction_dt))
from yourtable
group by affiliate_id

Or if they skip some months:

select affiliate_id,
sum(earnings)/count(distinct convert(varchar(7), transaction_dt, 120))
from yourtable
group by affiliate_id