Question

I have a SQL table where I need to work out the average time after each transactions:

The data looks like:

Tran1 07/09/2011 09:09:07  - CUSTOMER1 
Tran2 07/09/2011 09:30:46  - CUSTOMER1 
Tran3 07/09/2011 11:27:01  - CUSTOMER2 
Tran4 07/09/2011 11:29:22  - CUSTOMER2 
Tran5 07/09/2011 13:23:48  - CUSTOMER1 
Tran6 08/09/2011 14:21:29  - CUSTOMER3 
Tran7 08/09/2011 14:25:23  - CUSTOMER3 
Tran8 10/09/2011 13:28:57  - CUSTOMER1 
Tran9 10/09/2011 13:30:21  - CUSTOMER1 
Tran10 10/09/2011 13:49:13 - CUSTOMER4

The table is Transaction Table and there are three columns:-

ID = UniqueID, TimeStamp = DataTime, CustomerId = UniqueID

So if I pass in a parameter DateTime.. let say ’10/09/2011′
The result Im trying to achieve is..

Date:10/9/2011 AverageQueueTime:3mins2secs – for exmaple

Answer 1

Assuming your table is named “MyTable” (Is it really named Transaction!?) and that you want the difference in minutes:

SELECT  CustomerID ,
        SUM(timeSinceLastTransaction) / COUNT(*)
FROM    ( SELECT    * ,
                    DATEDIFF(MINUTE,
                             ( SELECT TOP 1
                                        t2.DataTime
                               FROM     MyTable t2
                               WHERE    t2.DataTime < t1.DataTime
                                        AND t2.CustomerId = t1.CustomerId
                               ORDER BY t2.DataTime DESC
                             ),
                             t1.DataTime
                             ) AS timeSinceLastTransaction
          FROM      MyTable t1
        ) AS IndividualTimes

This is a correlated subquery.