Show Each Month And Count Or 0 If Null [duplicate] - sql-server

Question

This question already has answers here:
SQL Date Query include month even if no data
(2 answers)
Closed 3 years ago.
I am using the below DDL to show sales data for months in 2018 and 2019. My issue is that obviously we will have no data for Sept - Dec 2019, but I need that to be returned and show a 0 amount for it.
How do I need to edit this query so that I have all months 1 - 12 for both 18 and 19 returned?
CREATE TABLE PrevYear (
[EmployeeNumber] char(8) NOT NULL,
[SaleAmount] int DEFAULT NULL,
[SaleDate] date NOT NULL,
[EmployeeName] char(17) NOT NULL
);
CREATE TABLE CurrentYear (
[EmployeeNumber] char(8) NOT NULL,
[SaleAmount] int DEFAULT NULL,
[SaleDate] date NOT NULL,
[EmployeeName] char(17) NOT NULL
);
INSERT INTO CurrentYear
VALUES ('ea12', '100', '2019-01-10', 'Sam Smith');
INSERT INTO CurrentYear
VALUES ('ea12', '199', '2019-01-13', 'Sam Smith');
INSERT INTO CurrentYear
VALUES ('ea12', '100', '2019-03-01', 'Sam Smith');
INSERT INTO CurrentYear
VALUES ('ls22', '100', '2019-05-01', 'Sam Smith');
INSERT INTO PrevYear
VALUES ('ea12', '100', '2018-01-10', 'Sam Smith');
INSERT INTO PrevYear
VALUES ('ea12', '199', '2018-01-13', 'Sam Smith');
INSERT INTO PrevYear
VALUES ('ea12', '100', '2018-03-01', 'Sam Smith');
INSERT INTO PrevYear
VALUES ('ls22', '100', '2018-05-01', 'Sam Smith');
My desired output from the query is:
Jan 18 $1
Jan 19 $1
Feb 18 $1
Feb 19 $1
Mar 18 $1
Mar 19 $1
Apr 18 $1
Apr 19 $1
May 18 $1
May 19 $1
Jun 18 $1
Jun 19 $1
Jul 18 $1
Jul 19 $1
Aug 18 $1
Aug 19 $1
Sep 18 $1
Sep 19 $0
Oct 18 $1
Oct 19 $0
Nov 18 $1
Nov 19 $0
Dec 18 $1
Dec 19 $0
This is the query I had
SELECT Month, Sum(ia) AS SaleAmount
FROM (SELECT Date_format(saledate, '%m-%Y') AS Month,
employeename,
Sum(SaleAmount) AS IA
FROM CurrentYear
WHERE employeename = 'Sam Smith'
GROUP BY Date_format(saledate, '%m-%Y'),
employeename
UNION ALL
SELECT Date_format(saledate, '%m-%Y') AS Month,
employeename,
Sum(SaleAmount) AS IA
FROM PrevYear
WHERE employeename = 'Sam Smith'
GROUP BY Date_format(saledate, '%m-%Y'),
employeename) previousQuery
GROUP BY month
ORDER BY month

Answer 1

You can simply add a lookup table with month that will contains 12 records (one for each month)
The structure will be: id, description.
On existing tables you can refers month by id: less space more performance in join.
On your query using left/right join you will always have the missing month.
PS: need only one table to manage this, and adding a simple where condition (or group by condition) where needed for years management