CTE Rolling 3 Mo Avg - sql

Question

output for all 3 queries
working on an assigment, below is the ask, she has directed us to use a CTE
Write SQL query code used to explore the database tables and write a query that retrieves finance amounts from "FactFinance" in the "AdventureWorksDW2016CTP3" database and returns those amounts, organized by month, and showing a 3-month rolling average
SELECT DateKey,
month(date) as [Month],
year(date) as [Year],
SUM ( ALL Amount) OVER (PARTITION BY Date ORDER BY Date ASC) AS Amount
FROM FactFinance
SELECT
YEAR(Date) AS Year,
MONTH(Date) AS Month,
SUM(Amount) AS Amount
FROM FactFinance
GROUP BY YEAR(Date), MONTH(Date)
ORDER BY Year, Month;
WITH CTE AS (
SELECT
DateKey AS Month,
AVG(Amount) AS AvgAmt
from FactFinance
group by DateKey
)
SELECT
Month,
AvgAmt
FROM CTE
GO
oUTPUT for last query Needing 3 month rolling average

Answer 1

First, you should know the right way to answer this. Assuming you have data for all three months, then:
SELECT YEAR(Date) AS Year,
MONTH(Date) AS Month,
SUM(Amount) AS Amount,
AVG(SUM(Amount)) OVER (ORDER BY MIN(DATE)
ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) as rolling_3month_avg
FROM FactFinance
GROUP BY YEAR(Date), MONTH(Date)
ORDER BY Year, Month;
If I were told to use a CTE for this, I might be tempted to do:
WITH unnecessary_cte as (
SELECT YEAR(Date) AS Year,
MONTH(Date) AS Month,
SUM(Amount) AS Amount,
AVG(SUM(Amount)) OVER (ORDER BY MIN(DATE)
ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) as rolling_3month_avg
FROM FactFinance
GROUP BY YEAR(Date), MONTH(Date)
)
SELECT *
FROM unnecessary_cte
ORDER BY YEAR, MONTH;
However, we can try to read your instructor's mind and speculate that she wants you to write something like this:
WITH ym as (
SELECT YEAR(Date) AS Year,
MONTH(Date) AS Month,
SUM(Amount) AS Amount
FROM FactFinance
GROUP BY YEAR(Date), MONTH(Date)
)
SELECT ym.*,
(SELECT AVG(Amount)
FROM ym ym2
WHERE 12 * ym2.year + ym2.month
BETWEEN 12 * ym.year + ym.month - 2 AND
12 * ym.year + ym.month
) as rolling_3month_avg
FROM ym
ORDER BY YEAR, MONTH;