Calculate Days between same date field in multiple records, looping for each record

Question

Be patient as I am a newbie to Sql Server (using 2008 R2) and SSIS.  I have looked at multiple solutions in this and many other forums before submitting the question.
I am looking at records to identify the number of days between two dates.  We review drug information.  A member uses drug A (Code identifies this called GPI) and has refills throughout time.  The goal is to calculate the days between Refill
1 and refill 2, then refill 2 to refill 3, etc.  When that member record set is complete, we move to the next member record set.
The basic data would be as follows.  Each record shown below would be a separate record in a single table :
Member
Drug Code
      DateService
         Days Between
1
A
3/1/2012
0
1
A
3/27/2012
27
1
A
4/21/2012
26
1
A
6/15/2012
56
1
F
2/25/2012
0
1
F
3/20/2012
25
1
F
4/17/2012
29
1
F
8/1/2012
107
5
B
2/15/2012
0
5
B
3/10/2012
25
5
B
4/10/2012
32
5
B
6/28/2012
80
5
B
7/10/2012
13
5
B
8/20/2012
42
The days between is the value I am looking to calculate.  The First record for the grouping of Member and Drug Code will be zero ( 0 ) as there is no preceding date to calculate against.  The process would then loop through the records at the grouping
(Member and Drug Code) until those records are calculated.  It would then move on to the next group of Member/Drug records.
Any assistance would be greatly appreciated.  Any clarification I can provide, I would be more than happpy too.  Thank you in advance!

Answer 1

TSQL- Finding the difference in days of multiple records in SQL ServerWITH OrderDates AS (
SELECT
ROW_NUMBER() OVER (ORDER BY OrderDate DESC) AS RowNumber,
OrderDate
FROM OrdersTable
WHERE SKU = 'AA0000'
SELECT
AVG(DATEDIFF(DD, O2.OrderDate, O1.OrderDate)) AS AverageFrequency
FROM OrderDates O1
LEFT JOIN OrderDates O2
ON O2.RowNumber = O1.RowNumber + 1
Fabio Bilicki