Nonlinear regression

[Jeremy]

I am no statistician and I don't have access to the data mining package in
SQL server. All I have is transact-sql. I want a simple trend of the growth
rate for a time series i.e.

x y rate
date1 sales1
date2 sales2 %rate2
date3 sales3 %rate3

I want to mimic the LOGEST function in Excel. I understand linear
regression well enough, but I didn't study nonlinear regression in school, so
I am learning what I can from the web, but it is slow going. This model
doesn't have to be too accurate, I am just looking for a reasonable estimate
of growth to help identify trends in customer purchase behavior.

I will ultimately calculate the growth rate for each customer for a given 12
month period.

Thanks.