Transformations > Expression transformation > Example: Use a window to calculate expiration dates

Example: Use a window to calculate expiration dates

You are a banker with information about the financial plans of two of your customers. Each plan has an associated start date.

For each customer, you want to know the expiration date for the current plan based on the activation date of the next plan. The previous plan ends when a new plan starts, so the end date for the previous plan is the start date of the next plan minus one day.

The following table lists the customer codes, the associated plan codes, and the start date of each plan:

CustomerCode	PlanCode	StartDate
C1	00001	2014-10-01
C2	00002	2014-10-01
C2	00002	2014-11-01
C1	00004	2014-10-25
C1	00001	2014-09-01
C1	00003	2014-10-10

To calculate expiration dates, complete the following tasks:

1Define partition and order keys.

You configure the following window properties to partition the data by customer code and order the data by ascending start date:

Property	Value	Description
Frame	Not specified.	The LEAD function will access rows based on the offset argument and ignore the frame.
Partition key	CustomerCode	Groups the rows according to customer code so that calculations are based on individual customers.
Order key	StartDate Ascending	Arranges the data chronologically by ascending start date.

The following table lists the data grouped by customer code and ordered by start date:

CustomerCode	PlanCode	StartDate
C1	00001	2014-09-01
C1	00002	2014-10-01
C1	00003	2014-10-10
C1	00004	2014-10-25
C2	00001	2014-10-01
C2	00002	2014-11-01

2Define a window function.

You define a LEAD function to access the subsequent row for every input.

You configure an expression field that performs the following calculation:

LEAD ( StartDate, 1, '01-Jan-2100' )

For more information about the LEAD function, see Function Reference.

3Define an ADD_TO_DATE function.

You use an ADD_TO_DATE function to subtract one day from the date you accessed.

You configure an expression field that performs the following calculation:

ADD_TO_DATE ( LEAD ( StartDate, 1, '01-Jan-2100' ), 'DD', -1, )

By subtracting one day from the start date of the next plan, you find the end date of the current plan.

The following table lists the end dates of each plan:

CustomerCode	PlanCode	StartDate	EndDate
C1	00001	2014-09-01	2014-09-30
C1	00002	2014-10-01	2014-10-09
C1	00003	2014-10-10	2014-10-24
C1	00004	2014-10-25	2099-12-31*
C2	00001	2014-10-01	2014-10-31
C2	00002	2014-11-01	2099-12-31*

*The LEAD function returned the default value because these plans have not yet ended. The rows were outside the partition, so the ADD_TO_DATE function subtracted one day from 01-Jan-2100, returning 2099-12-31.