有两种选择：
使用联接
使用变量
使用join的方法如下：

SELECT
       t1.Tgrowth,
       sum(t2.Tgrowth) as Emp_Count,
       CASE
        WHEN t1.Month = 1 THEN 'JAN'
        WHEN t1.Month = 2 THEN 'FEB'
        WHEN t1.Month = 3 THEN 'MAR'
        WHEN t1.Month = 4 THEN 'APR'
        WHEN t1.Month = 5 THEN 'MAY'
        WHEN t1.Month = 6 THEN 'JUN'
        WHEN t1.Month = 7 THEN 'JUL'
        WHEN t1.Month = 8 THEN 'AUG'
        WHEN t1.Month = 9 THEN 'SEP'
        WHEN t1.Month = 10 THEN 'OCT'
        WHEN t1.Month = 11 THEN 'NOV'
        WHEN t1.Month = 12 THEN 'DEC'
       END AS myMONTH
FROM (
    SELECT
    case
        when r.growth is not null then r.growth
        when r.growth is null then 0
    END as Tgrowth,
    t.mon_num AS Month
FROM
    (SELECT 1 mon_num UNION SELECT 2 UNION SELECT 3 UNION SELECT 4
    UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8
    UNION SELECT 9 UNION SELECT 10 UNION SELECT 11 UNION SELECT 12) t
LEFT JOIN Reports r ON t.mon_num = r.themonth
    AND r.theYear = 1993
GROUP BY r.growth , Month
ORDER BY t.mon_num ASC
         ) as t1 join (
    SELECT
    case
        when r.growth is not null then r.growth
        when r.growth is null then 0
    END as Tgrowth,
    t.mon_num AS Month
FROM
    (SELECT 1 mon_num UNION SELECT 2 UNION SELECT 3 UNION SELECT 4
    UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8
    UNION SELECT 9 UNION SELECT 10 UNION SELECT 11 UNION SELECT 12) t
LEFT JOIN Reports r ON t.mon_num = r.themonth
    AND r.theYear = 1993
GROUP BY r.growth , Month
ORDER BY t.mon_num ASC
         ) as t2 on t1.Month >= t2.Month group by t1.Month;

使用变量解决方案如下：

SET @num := 0;
select
       Tgrowth,
       @num := @num + Tgrowth as Emp_Count,
       CASE
        WHEN t1.Month = 1 THEN 'JAN'
        WHEN t1.Month = 2 THEN 'FEB'
        WHEN t1.Month = 3 THEN 'MAR'
        WHEN t1.Month = 4 THEN 'APR'
        WHEN t1.Month = 5 THEN 'MAY'
        WHEN t1.Month = 6 THEN 'JUN'
        WHEN t1.Month = 7 THEN 'JUL'
        WHEN t1.Month = 8 THEN 'AUG'
        WHEN t1.Month = 9 THEN 'SEP'
        WHEN t1.Month = 10 THEN 'OCT'
        WHEN t1.Month = 11 THEN 'NOV'
        WHEN t1.Month = 12 THEN 'DEC'
       END AS myMONTH
from (
SELECT
    case
        when r.growth is not null then r.growth
        when r.growth is null then 0
    END as Tgrowth,
    t.mon_num AS Month
FROM
    (SELECT 1 mon_num UNION SELECT 2 UNION SELECT 3 UNION SELECT 4
    UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8
    UNION SELECT 9 UNION SELECT 10 UNION SELECT 11 UNION SELECT 12) t
LEFT JOIN Reports r ON t.mon_num = r.themonth
    AND r.theYear = 1993
GROUP BY r.growth , Month
ORDER BY t.mon_num ASC ) t1;

因为您运行的是mysql 8.0，所以我建议使用递归查询来生成日期，然后使用窗口函数和聚合。
如果你想要整个1993年：

with dates as (
    select '1993-01-01' dt
    union all
    select dt + interval 1 month from dates where dt < '1993-12-01'
)
select
    date_format(d.dt, '%b') mymonth,
    coalesce(sum(started), 0) - coalesce(sum(ended), 0) growth,
    sum(coalesce(sum(started), 0) - coalesce(sum(ended), 0)) over(order by d.dt) emp_count
from dates d
left join reports r on r.theDate >= d.dt and r.theDate < d.dt + interval 1 month
group by d.dt
order by d.dt

这是假设 theDate 存储为 date 数据类型而不是字符串（否则，您需要首先使用 str_to_date() ).
这还考虑到表可能包含给定月份的多行。如果不是这样，那么就不需要聚合：

with dates as (
    select '1993-01-01' dt
    union all
    select dt + interval 1 month from dates where dt < '1993-12-01'
)
select
    date_format(d.dt, '%b') mymonth,
    coalesce(started, 0) - coalesce(ended, 0) growth,
    sum(coalesce(started, 0) - coalesce(ended, 0)) over(order by d.dt) emp_count
from dates d
left join reports r on r.theDate >= d.dt and r.theDate < d.dt + interval 1 month
order by d.dt