Over and Partition by on SQL for aggregation

The other day  I had a requirement to create a report with total occurrence of an event by year month , running total and then running total within a year all in the same result set.  After trying out different methods, I found the results can be easily obtained by using the DB2’s OLAP function  OVER and PARTITION BY

OVER( ) by lets you do aggregate  to a subset of data.PARTITION BY let you split/segregate the subset of data  Here is a basic example of the OVER(PARTITION BY)

select dept,avg(salary) over (partition by dept) from employee_salary order by dept;

This will give the average salary of each department

An example of the result set I was after and  below is the sql that provided this result set :

In this sql I used  the OVER() and then the OVER(PARTITION BY) OLAP function.  The OVER(ORDER BY yearname,monthname) give me the running sum of event_count for the ongoing months. The
over (partition by yearname order by yearname,monthname) give me the running sum of event_count for ongoing month but reset itself at the beginning of every year.

SELECT YEARNAME, MONTHNAME, mytab.count as EVENT_COUNT,
SUM(mytab.count) OVER (ORDER BY yearname,monthname) AS ROLLING_SUM,
SUM(mytab.count) OVER (PARTITION BY yearname ORDER BY yearname,monthname) as YEARLY_SUM
FROM
(select year(A.add_dt) as yearname,
month(A.add_dt) monthname ,
count(1) count
from raju.hack_attempts A
where year(A.add_dt) in (2007, 2008,2009)
group by year(A.add_dt),
month(A.add_dt)
)mytab