Question

I’ve an issue where I am trying to pull identify customer orders where

1. The customer had an order in the same month one year ago and this
   month.
2. The customer had an order one year ago this month but not this month
3. The customer had an order this month but not one year ago.

My data looks like this.

CustomerID      Month   Year    Value
1                 2     2011       18
1                 2     2012       16
2                 1     2011       15
3                 1     2012       13

From this I would expect to return (for each criteria above)

1. Rows 1 & 2
2. Row 3
3. Row 4

I’m sure there must be an elegant solution to this but for the life of me I can’t see it today!

So far I’ve got to this but I get a lot of duplicates am I missing something obvious?

;with t (ID, Yr, Mo) 
As
(
    Select ID, TransactionYearFK, TransactionMonthFK
    From dbo.MyData
)

Select * 
From t tC
        Full Outer Join t tL On tC.ID = tL.ID And tC.Mo = tL.Mo And tL.Yr = tC.Yr - 1

Where Coalesce(tC.ID, tL.ID) = 21110
Order By tC.Yr, tC.Mo

Answer 1

If you want all the results in one query, your conditions could be restated like this:

1. The year must be no earlier than the last one.

2. The month must be the same as the current month, whether this or the last year.

Assuming then you want to return order counts per month in each year for every customer, you could try something like this:

SELECT
  CustomerID,
  ThisYear = COUNT(CASE Year WHEN YEAR(GETDATE()) THEN 1 ELSE NULL END),
  LastYear = COUNT(CASE Year WHEN YEAR(GETDATE()) THEN NULL ELSE 1 END)
FROM data
WHERE Year >= YEAR(GETDATE()) - 1
  AND Month = MONTH(GETDATE())
GROUP BY
  CustomerID