SQL calculating stock per month - postgresql

Question

I have specific task, and don't know how to realize it. I hope someone can help me =)
I have stock_move table:
product_id |location_id |location_dest_id |product_qty |date_expected |
-----------|------------|-----------------|------------|--------------------|
327 |80 |84 |10 |2014-05-28 00:00:00 |
327 |80 |84 |10 |2014-05-23 00:00:00 |
327 |80 |84 |10 |2014-02-26 00:00:00 |
327 |80 |85 |10 |2014-02-21 00:00:00 |
327 |80 |84 |10 |2014-02-12 00:00:00 |
327 |84 |85 |20 |2014-02-06 00:00:00 |
322 |84 |80 |120 |2015-12-16 00:00:00 |
322 |80 |84 |30 |2015-12-10 00:00:00 |
322 |80 |84 |30 |2015-12-04 00:00:00 |
322 |80 |84 |15 |2015-11-26 00:00:00 |
i.e. it's table of product moves from one warehouse to second.
I can calculate stock at custom date if I use something like this:
select
coalesce(si.product_id, so.product_id) as "Product",
(coalesce(si.stock, 0) - coalesce(so.stock, 0)) as "Stock"
from
(
select
product_id
,sum(product_qty * price_unit) as stock
from stock_move
where
location_dest_id = 80
and date_expected < now()
group by product_id
) as si
full outer join (
select
product_id
,sum(product_qty * price_unit) as stock
from stock_move
where
location_id = 80
and date_expected < now()
group by product_id
) as so
on si.product_id = so.product_id
Result I have current stock:
Product |Stock |
--------|------|
325 |1058 |
313 |34862 |
304 |2364 |
BUT what to do if I need stock per month?
something like this?
Month |Total Stock |
--------|------------|
Jan |130238 |
Feb |348262 |
Mar |2323364 |
How can I sum product qty from start period to end of each month?
I have just one idea - it's use 24 sub queries for get stock per each month (ex. below)
Jan |Feb | Mar |
----|----|-----|
123 |234 |345 |
End after this rotate rows and columns?
I think this's stupid, but I don't know another way... Help me pls =)

Answer 1

Something like this could give you monthly "ending" inventory snapshots. The trick is your data may omit certain months for certain parts, but that part will still have a balance (ie 50 received in January, nothing happened in February, but you still want to show February with a running total of 50).
One way to handle this is to come up with all possible combinations part/dates. I assumed 1/1/14 + 24 months in this example, but that's easily changed in the all_months subquery. For example, you may only want to start with the minimum date from the stock_move table.
with all_months as (
select '2014-01-01'::date + interval '1 month' * generate_series(0, 23) as month_begin
),
stock_calc as (
select
product_id, date_expected,
date_trunc ('month', date_expected)::date as month_expected,
case
when location_id = 80 then -product_qty * price_unit
when location_dest_id = 80 then product_qty * price_unit
else 0
end as qty
from stock_move
union all
select distinct
s.product_id, m.month_begin::date, m.month_begin::date, 0
from
stock_move s
cross join all_months m
),
running_totals as (
select
product_id, date_expected, month_expected,
sum (qty) over (partition by product_id order by date_expected) as end_qty,
row_number() over (partition by product_id, month_expected
order by date_expected desc) as rn
from stock_calc
)
select
product_id, month_expected, end_qty
from running_totals
where
rn = 1