Get NULL value when using an aggregate function - sql

Question

Here is the tables:
https://dbfiddle.uk/markdown?rdbms=sqlserver_2019&fiddle=effc94afe681b2dfdb3e2c02c2b005ea
I want to find the average Total Amount for last 3 values (I mean the last 3 OrderID) for each customer. If customer doesn't have 3 operation, result should be null.
Here is my answer (T-SQL):
SELECT s.CustomerID,avg(s.TotalAmount) as AverageofLast3_operation
FROM (SELECT OrderID, CustomerID, EventDate, TotalAmount,
ROW_NUMBER() over (partition by CustomerID ORDER BY OrderID asc) as Row_num
FROM CustomerOperation
)s
WHERE s.Row_num>3
GROUP BY CustomerID
And the result is:
CustomerID
AverageofLast3_operation
1
7833
2
1966
According to the question, I should also have a row like this:
CustomerID
AverageofLast3_operation
3
NULL
How can I achieve this with T-SQL?

Answer 1

You need conditional aggregation:
SELECT CustomerID,
AVG(CASE WHEN counter >= 3 THEN TotalAmount END) AS AverageofLast3_operation
from (
SELECT OrderID, CustomerID, EventDate, TotalAmount,
ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY OrderID DESC) AS Row_num,
COUNT(*) OVER (PARTITION BY CustomerID) counter
FROM CustomerOperation
) s
WHERE Row_num <= 3
GROUP BY CustomerID;
Or:
SELECT CustomerID,
CASE WHEN COUNT(*) = 3 THEN AVG(TotalAmount) END AS AverageofLast3_operation
from (
SELECT OrderID, CustomerID, EventDate, TotalAmount,
ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY OrderID DESC) AS Row_num
FROM CustomerOperation
) s
WHERE Row_num <= 3
GROUP BY CustomerID;
See the demo.

Answer 2

You can use a conditional average like so:
with t as (
select customerId,
case when
Row_Number() over(partition by customerid order by orderid desc) <=3 then totalamount
else 0 end TotalAmount,
Count(*) over (partition by customerid) cnt
from CustomerOperation
)
select customerId, Avg(case when cnt>=3 then totalamount end) as Average
from t
where totalAmount>0
group by CustomerId