query to display additional column based on aggregate value - sql

Question

I've been mulling on this problem for a couple of hours now with no luck, so I though people on SO might be able to help :)
I have a table with data regarding processing volumes at stores. The first three columns shown below can be queried from that table. What I'm trying to do is to add a 4th column that's basically a flag regarding if a store has processed >=$150, and if so, will display the corresponding date. The way this works is the first instance where the store has surpassed $150 is the date that gets displayed. Subsequent processing volumes don't count after the the first instance the activated date is hit. For example, for store 4, there's just one instance of the activated date.
store_id sales_volume date activated_date
----------------------------------------------------
2 5 03/14/2012
2 125 05/21/2012
2 30 11/01/2012 11/01/2012
3 100 02/06/2012
3 140 12/22/2012 12/22/2012
4 300 10/15/2012 10/15/2012
4 450 11/25/2012
5 100 12/03/2012
Any insights as to how to build out this fourth column? Thanks in advance!

Answer 1

The solution start by calculating the cumulative sales. Then, you want the activation date only when the cumulative sales first pass through the $150 level. This happens when adding the current sales amount pushes the cumulative amount over the threshold. The following case expression handles this.
select t.store_id, t.sales_volume, t.date,
(case when 150 > cumesales - t.sales_volume and 150 <= cumesales
then date
end) as ActivationDate
from (select t.*,
sum(sales_volume) over (partition by store_id order by date) as cumesales
from t
) t
If you have an older version of Postgres that does not support cumulative sum, you can get the cumulative sales with a subquery like:
(select sum(sales_volume) from t t2 where t2.store_id = t.store_id and t2.date <= t.date) as cumesales

Answer 2

Variant 1
You can LEFT JOIN to a table that calculates the first date surpassing the 150 $ limit per store:
SELECT t.*, b.activated_date
FROM tbl t
LEFT JOIN (
SELECT store_id, min(thedate) AS activated_date
FROM (
SELECT store_id, thedate
,sum(sales_volume) OVER (PARTITION BY store_id
ORDER BY thedate) AS running_sum
FROM tbl
) a
WHERE running_sum >= 150
GROUP BY 1
) b ON t.store_id = b.store_id AND t.thedate = b.activated_date
ORDER BY t.store_id, t.thedate;
The calculation of the the first day has to be done in two steps, since the window function accumulating the running sum has to be applied in a separate SELECT.
Variant 2
Another window function instead of the LEFT JOIN. May of may not be faster. Test with EXPLAIN ANALYZE.
SELECT *
,CASE WHEN running_sum >= 150 AND thedate = first_value(thedate)
OVER (PARTITION BY store_id, running_sum >= 150 ORDER BY thedate)
THEN thedate END AS activated_date
FROM (
SELECT *
,sum(sales_volume)
OVER (PARTITION BY store_id ORDER BY thedate) AS running_sum
FROM tbl
) b
ORDER BY store_id, thedate;
->sqlfiddle demonstrating both.