I've been working on this and I don't think I'm doing it right. |D

Our database doesn't keep track of how many customers we retain so we looked for an alternate method. It's outlined in this article. It suggests you have this table to fill in:

Year  Number of Customers   Number of customers Retained in 2009    Percent (%) Retained in 2009    Number of customers Retained in 2010    Percent (%) Retained in 2010  ....

2008
2009
2010
2011
2012
Total

The table would go out to 2012 in the headers. I'm just saving space.

It tells you to find the total number of customers you had in your starting year. To do this, I used this query since our starting year is 2008:

select YEAR(OrderDate) as 'Year', COUNT(distinct(billemail)) as Customers
from dbo.tblOrder
where OrderDate >= '2008-01-01' and OrderDate <= '2008-12-31'
group by YEAR(OrderDate)

At the moment we just differentiate our customers by email address.

Then you have to search for the same names of customers who purchased again in later years (ours are 2009, 10, 11, and 12).

I came up with this. It should find people who purchased in both 2008 and 2009.

SELECT YEAR(OrderDate) as 'Year',COUNT(distinct(billemail)) as Customers
FROM dbo.tblOrder o with (nolock)
WHERE o.BillEmail IN (SELECT DISTINCT o1.BillEmail 
        FROM dbo.tblOrder o1 with (nolock)
        WHERE o1.OrderDate BETWEEN '2008-1-1' AND '2009-1-1')
    AND o.BillEmail IN (SELECT DISTINCT o2.BillEmail 
        FROM dbo.tblOrder o2 with (nolock)
        WHERE o2.OrderDate BETWEEN '2009-1-1' AND '2010-1-1')
    --AND o.OrderDate BETWEEN '2008-1-1' AND '2013-1-1' 
    AND o.BillEmail NOT LIKE '%@halloweencostumes.com'
    AND o.BillEmail NOT LIKE ''
GROUP BY YEAR(OrderDate)

So I'm just finding the customers who purchased in both those years. And then I'm doing an independent query to find those who purchased in 2008 and 2010, then 08 and 11, and then 08 and 12. This one finds 2008 and 2010 purchasers:

SELECT YEAR(OrderDate) as 'Year',COUNT(distinct(billemail)) as Customers
FROM dbo.tblOrder o with (nolock)
WHERE o.BillEmail IN (SELECT DISTINCT o1.BillEmail 
        FROM dbo.tblOrder o1 with (nolock)
        WHERE o1.OrderDate BETWEEN '2008-1-1' AND '2009-1-1')
    AND o.BillEmail IN (SELECT DISTINCT o2.BillEmail 
        FROM dbo.tblOrder o2 with (nolock)
        WHERE o2.OrderDate BETWEEN '2010-1-1' AND '2011-1-1')
    --AND o.OrderDate BETWEEN '2008-1-1' AND '2013-1-1' 
    AND o.BillEmail NOT LIKE '%@halloweencostumes.com'
    AND o.BillEmail NOT LIKE ''
GROUP BY YEAR(OrderDate)

So you see I have a different query for each year comparison. They're all unrelated. So in the end I'm just finding people who bought in 2008 and 2009, and then a potentially different group that bought in 2008 and 2010, and so on. For this to be accurate, do I have to use the same grouping of 2008 buyers each time? So they bought in 2009 and 2010 and 2011, and 2012?

This is where I'm worried and not sure how to proceed or even find such data.

Any advice would be appreciated! Thanks!