oracle's analytical function issue - oracle

Question

Please let me know if the following is off topic, or not clear, or too specific, or too complex to understand. I think the following is a challenge to describe, understand and solve.
CIF=cost, insurance, frieght (basically it is the import value)
The simiplified version of input table (Import) looks like this:
enter image description here
So from January to June the value 1 is assigned to SixMonthPeriod column, and the rest of the months are given the value 2.
I then want to calculate unit price for a six period, thus I use
select SixMonthPeriod, ProductDescrip, Sum(weight), Sum (CIF), (Sum (CIF))/(Sum(weight)) as UnitPrice
from Import
group by SixMonthPeriod, ProductDescrip;
This is fine, but I then want to calculate inflation for each product (over a six month period )where I need to use lag (an oracle analytical function). The six month period has to be fixed. Thus, if the previous period for a particular product is missing, then the unit price should be zero. I want to re-begin/begin the calculation of inflation for each product. The unit price and inflation equations looks like the following, respectively:
unit price = (Sum(weight) over a six month period)/(Sum (CIF) over a six month period)
inflation = (Current Unit price - previous unit price)/(previous unit price)
I use the following SQL to calculate inflation for a six month period for each product, where the calculation begins again for each product:
select Yr, SixMthPeriod, Product, UnitPrice, LagUnitPrice, ((UnitPrice -LagUnitPrice)/LagUnitPrice) as inflation
from (select Year as Yr, SixMonthPeriod as SixMthPeriod,
ProductDescrip as product, (Sum (CIF))/(Sum(weight)) as UnitPrice,
lag((Sum (CIF))/(Sum(weight)))
over (partition by ProductDescrip order by YEAR, SixMonthPeriod) as LagUnitPrice
From Import
group by Year, SixMonthPeriod, ProductDescrip)
The problem is the inflation period is not fixed.
For example, for the result, I get the following:
enter image description here
The first two rows are fine and there should be null values because they are my first line, thus there is no LagUnitPrice and inflation.
The third line has a problem where it has taken 0.34 as the LagUnitPrice but actually it is zero (for the period 2016 where SixMthPeriod=1 for the product barley). the oracle analytical functions does not take into account missing rows (e.g. for the period 2016 where SixMthPeriod=1 for the product barley).
how do I fix this problem (if you understand me)?
I have 96 rows, thus I can export the file to excel, and use excel's formulas to fix these exceptions.

Answer 1

You can autogenerate missing periods with nullable price, attach them to your data and do the rest as you did:
select product, year, smp, price, prev_price, (price - prev_price) / prev_price inflation
from (
select product, year, smp, price,
lag(price) over (partition by product order by year, smp) prev_price
from (
select year, ProductDescrip product, SixMonthPeriod smp, sum(CIF)/sum(weight) price
from Import
group by year, SixMonthPeriod, ProductDescrip) a
full join (
select distinct year, productdescrip product, column_value smp
from import cross join table(sys.odcinumberlist(1, 2))) b
using (product, year, smp))
order by product, year, smp
SQLFiddle demo
Subquery b is responsible for generating all periods, you can run it separately to see what it produces.