Year over Year by Month Comparison and Month to Date in BigQuery - sql

Question

Edit: #shawnt00 has the correct answer. Thank you very much!
I am having trouble accurately doing a year over year comparison by month but at any point during the month. For example for August 2022 vs 2021, I want to compare August 1 - August 25, rather than full month of August 2021.
I am also using a daily date field.
I want the final result to basically be:
Product_ID, Year, Month, PY_Sales, CY_Sales
Edit: I have daily totals. Some products do have not sales on certain days though:
product_id
sale_date
units
1
2021-01-01
5
2
2021-01-02
4
...
...
...
1
2021-06-05
2
2
2022-01-06
1
2
2022-08-15
9
This is the code I have, but it doesn't do MTD. So 2021 August is the entire month of August and I want it the same dates for 2022. I used this code because some products do not have sales on certain months.
WITH cte AS
(
SELECT
PRODUCT_ID,
EXTRACT(YEAR FROM SALE_DATE) AS Year,
EXTRACT(MONTH FROM SALE_DATE) AS Month,
CONCAT(EXTRACT(YEAR FROM SALE_DATE), '-',EXTRACT(MONTH FROM SALE_DATE)) AS Year_Month,
SUM(Units) AS Units
FROM data
WHERE Product_ID = 1
AND DATE(SALE_DATE) >= '2019-01-01'
GROUP BY 1, 2, 3
),
diff AS
(
SELECT
COALESCE(c.PRODUCT_ID, p.PRODUCT_ID) AS Product_ID,
COALESCE(c.Year, p.Year + 1) AS Year,
COALESCE(c.Month, p.Month) AS Month,
IFNULL(c.Units, 0) AS Current_Units,
IFNULL(p.Units, 0) AS Previous_Units,
NULLIF(((IFNULL(c.Units, 0) - IFNULL(p.Units,0)) / p.Units),0) * 100 AS Percent_Change
FROM CTE c
FULL OUTER JOIN CTE p ON c.PRODUCT_ID = p.PRODUCT_ID AND c.Year = p.Year + 1 AND c.Month = p.Month
WHERE c.Year <= EXTRACT(YEAR FROM CURRENT_DATE())
ORDER BY 2, c.Year, c.Month
)
SELECT *
FROM diff
--This is to avoid dividing by 0
WHERE diff.Previous_Units > 0
--AND Percent_Change <= -.5

Answer 1

You could just roll up two different monthly totals and then switch for the current month comparison:
with agg as (
select
PRODUCT_ID,
extract(year from SALE_DATE) as yr,
extract(month from SALE_DATE) as mth,
sum(Units) as Units,
sum(case when extract(day from SALE_DATE) <= extract(day from current_date())
then Units end) as UnitsMTD
from data
where date(SALE_DATE) >= '2019-01-01' -- one year before report output
group by 1, 2, 3
)
select c.Yr, c.Mth, c.PRODUCT_ID,
case when Yr = extract(year from current_date())
and Mth = extract(month from current_date())
then (c.UnitsMTD - p.UnitsMTD) / p.UnitsMTD
else (c.Units - p.Units ) / p.Units
end as Percent_Change
from agg c left outer join agg p
on p.Product_ID = c.Product_ID and p.Yr = c.Yr - 1 and p.Mth = c.Mth
order by c.Yr, c.Mth, c.PRODUCT_ID;
Note my earlier comment about leap years. This will treat February 28 of the year following a leap year as an "MTD" month. You might need to handle that differently inside the case expression.