sql running total math current quarter - sql

Question

Im trying to figure out the total for the quarter when the only data shown is a running total for the year:
Id Amount Periods Year Type Date
-------------------------------------------------------------
1 65 2 2014 G 4-1-12
2 75 3 2014 G 7-1-12
3 25 1 2014 G 1-1-12
4 60 1 2014 H 1-1-12
5 75 1 2014 Y 1-1-12
6 120 3 2014 I 7-1-12
7 30 1 2014 I 1-1-12
8 90 2 2014 I 4-1-12
In the data shown above. The items in type G and I are running totals for the period (in qtrs). If my query returns period 3, is there a sql way to get the data for the qtr? The math would involve retrieving the data for the 3rd period - 2nd period.
Right now my sql is something like:
SELECT * FROM data WHERE Date='4-1-12';
In this query, it will return row #1, which is a total for 2 periods. I would like it to return just the total for the 2nd period. Im looking to make this happen with SQLite.
Any help would be appreciated.
Thank alot

Answer 1

You want to subtract the running total of the previous quarter:
SELECT Id,
Year,
Type,
Date,
Amount - IFNULL((SELECT Amount
FROM data AS previousQuarter
WHERE previousQuarter.Year = data.year
AND previousQuarter.Type = data.Type
AND previousQuarter.Periods = data.Periods - 1
), 0) AS Amount
FROM data
The IFNULL is needed to handle a quarter that has no previous quarter.