Oracle SQL - Comparing Multiple Rows with Dates

Question

I'm working in Oracle SQL. Suppose I have a table that lists the following.

TABLE

PurchaseID    CustID      Location       Date  
    1            1           A        8/23/2013 12:00:00 AM  
    2            1           B        8/15/2013 12:00:00 AM  
    3            2           A        5/15/2013 12:00:00 AM  
    4            2           B        1/01/2005 12:00:00 AM  
    5            3           A        1/15/2001 12:00:00 AM  
    6            3           A        1/30/2001 12:00:00 AM  
    7            3           B        8/23/2013 12:00:00 AM  
    8            4           A        5/05/2012 12:00:00 AM
    9            4           B        8/15/2010 12:00:00 AM
    10           4           A        9/20/2008 12:00:00 AM  

I'm trying to write a query that compares the purchases by customer so that the output is every instance where a particular customer makes a purchase at two different locations within 2 years of each other. I'm getting particularly tripped up on the CustID=3 and CustID=4 type cases, where there are difficult combinations of location/date. The output of the query should look like this.

PurchaseID    CustID      Location       Date  
    1            1           A        8/23/2013 12:00:00 AM  
    2            1           B        8/15/2013 12:00:00 AM   
    8            4           A        5/05/2012 12:00:00 AM
    9            4           B        8/15/2010 12:00:00 AM
    10           4           A        9/20/2008 12:00:00 AM

In the Output, CustID=1's purchases are returned because they are in different locations within 2 years of each other. CustID=2 are thrown out because they are not within 2 years. CustID=3 has two purchases within 2 years of each other, but they are thrown out because they are in the same location. And CustID=4's purchases are kept because Purchases 8 and 9 are within 2 years and in different locations, and 9 and 10 are within 2 years and in different locations (I want these to be kept despite 8 and 10 being in the same location and not within 10 years).

Note: The Date column has the Oracle SQL 'Date' Datatype.

As always, any help/guidance would be greatly appreciated.

Answer 1

You can limit the search to the cases where either the next or previous location is different from the current one. Then look at the time difference to choose the rows.

This makes heavy use of lag() and lead() :

select t.PruchaseId, t.CustId, t.Location, t.Date
from (select t.*,
             lag(location) over (partition by CustId order by Date) as prevloc,
             lead(location) over (partition by CustId order by Date) as nextloc,
             lag(date) over (partition by CustId order by Date) as prevdate,
             lead(date) over (partition by CustId order by Date) as nextdate
      from t
     ) t
where ((prevloc <> location) and (add_months(prevdate, 2*12) > date)) or
      ((nextloc <> location) and (add_months(date, 2*12) > nextdate));