MySQL 8.0

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WITH RECURSIVE
cte_months_to_pull AS (
    SELECT DATE_FORMAT(@start_date, '%Y-%m-01')
         - INTERVAL @number_of_months MONTH AS month_to_pull
    UNION ALL
    SELECT month_to_pull + INTERVAL 1 MONTH
    FROM cte_months_to_pull
    WHERE month_to_pull < @start_date + INTERVAL @number_of_months - 2 MONTH
)
SELECT YRS.months_to_pull
      ,T.employeename
      ,COALESCE(T.IA, 0) IA
FROM (SELECT DATE_Format(month_to_pull, '%b-%Y') months_to_pull
      FROM cte_months_to_pull
      ORDER BY months_to_pull
     ) AS YRS
LEFT JOIN (SELECT Date_format(saledate, '%b-%Y') AS `Month`
                 ,employeename
                 ,Sum(saleamount) AS IA
           FROM   CurrentYear
           WHERE  employeename = 'Maggie Samuels'
           GROUP  BY Date_format(saledate, '%b-%Y'), employeename
           UNION ALL
           SELECT Date_format(saledate, '%b-%Y')
                 ,employeename
                 ,Sum(saleamount)
           FROM   PrevYear
           WHERE  employeename = 'Maggie Samuels'
           GROUP  BY Date_format(saledate, '%b-%Y'), employeename) T
ON YRS.months_to_pull = T.`Month`
order by month(STR_TO_DATE(CONCAT('01-',months_to_pull), '%d-%b-%Y'))
        ,YEAR(STR_TO_DATE(CONCAT('01-',months_to_pull), '%d-%b-%Y'))

months_to_pull	employeename	IA
Jan-2018	Maggie Samuels	100
Jan-2019	Maggie Samuels	100
Feb-2018	null	0
Feb-2019	null	0
Mar-2018	null	0
Mar-2019	null	0
Apr-2018	null	0
Apr-2019	null	0
May-2018	Maggie Samuels	100
May-2019	Maggie Samuels	100
Jun-2018	null	0
Jun-2019	null	0
Jul-2018	null	0
Jul-2019	null	0
Aug-2018	null	0
Aug-2019	null	0
Sep-2018	null	0
Sep-2019	null	0
Oct-2018	null	0
Oct-2019	null	0
Nov-2018	null	0
Nov-2019	null	0
Dec-2018	null	0
Dec-2019	null	0