Removing Null results from AVG - sql

Question

I've been doing some work regarding AVG. My previous solution didn't work. However, after some revision and help it's now returning the correct results. The results do however, keep returning NULL Values for agents as they don't have results in the CASE WHEN date range.
I've tried adding IS NOT NULL into CASE WHEN argument but it tells me the expression does not exist.
SELECT Employee,
AVG(CASE WHEN SaleDate >= '2019-01-01' AND SaleDate < '2019-04-01' THEN NewScheme END),
AVG(CASE WHEN SaleDate >= '2019-01-04' AND SaleDate < '2019-04-07' THEN NewScheme END),
FROM Salereport
WHERE Business Area = 'Sales'
GROUP BY Employee;
When adding is not null i'm told the expression doesn't exist.

Answer 1

You can simply add a WHERE clause to remove employees that do not have sales in either range. It could still show NULL in one of the average columns though:
SELECT Employee,
AVG(CASE WHEN SaleDate >= '2019-01-01' AND SaleDate < '2019-04-01' THEN NewScheme END),
AVG(CASE WHEN SaleDate >= '2019-01-04' AND SaleDate < '2019-04-07' THEN NewScheme END)
FROM Salereport
WHERE Business Area = 'Sales'
AND SaleDate >= '2019-01-01' AND SaleDate < '2019-04-07'
-- if the ranges do not overlap then list them separately and combine with "OR"
GROUP BY Employee;

Answer 2

Build your query like this. Put ELSE 0 End and ISNULL() to avoid nulls your your AVG
SELECT Employee,
AVG(ISNULL(CASE WHEN SaleDate >= '2019-01-01' AND SaleDate < '2019-04-01'
THEN NewScheme ELSE 0 End, 0)
END),
AVG(ISNULL((CASE WHEN SaleDate >= '2019-01-04' AND SaleDate < '2019-04-07'
THEN NewScheme ELSE 0 End, 0)
END),
FROM Salereport
WHERE Business Area = 'Sales'
GROUP BY Employee;
OR
SELECT Employee,
AVG(CASE WHEN SaleDate >= '2019-01-01' AND SaleDate < '2019-04-01'
THEN ISNULL(NewScheme, 0) ELSE 0 End
END),
AVG((CASE WHEN SaleDate >= '2019-01-04' AND SaleDate < '2019-04-07'
THEN ISNULL(NewScheme, 0) ELSE 0 End
END),
FROM Salereport
WHERE Business Area = 'Sales'
GROUP BY Employee;