您可以使用子查询来构建月份列表，并将左键连接到数据表中

select monthname( t.my_date) monthname
  , ifnull(count(mount(date_plan)),0) cplan
  , ifnull(count(month(date_plan)),0) cstart
from (
  select str_to_date('2018-01-01', '%Y-%m-%d') as my_date 
  union 
  select str_to_date('2018-02-01', '%Y-%m-%d')
  union 
  select str_to_date('2018-03-01', '%Y-%m-%d')
  union 
  select str_to_date('2018-04-01', '%Y-%m-%d') 
  union 
  select str_to_date('2018-05-01', '%Y-%m-%d')
  union 
  select str_to_date('2018-06-01', '%Y-%m-%d')
  union 
  select str_to_date('2018-07-01', '%Y-%m-%d')
  union 
  select str_to_date('2018-08-01', '%Y-%m-%d')
  union 
  select str_to_date('2018-09-01', '%Y-%m-%d')
  union 
  select str_to_date('2018-10-01', '%Y-%m-%d') 
  union 
  select str_to_date('2018-11-01', '%Y-%m-%d')
  union 
  select str_to_date('2018-12-01', '%Y-%m-%d')
) t  
left join  data d  on month(d.date_plan) = month(t.my_date)
group by monthname

要得到想要的结果，首先必须创建一个月表，然后必须 LEFT JOIN 对于包含 data_plan 以及 data_start 每月：

SELECT MONTHNAME(CONCAT('2018-', m.month, '-01')) AS month, 
       COALESCE(cplan, 0) AS cplan,
       COALESCE(cstart, 0) AS cstart
FROM (SELECT 1 AS month 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) m
LEFT JOIN (SELECT MONTH(data_plan) AS month, COUNT(*) AS cplan
           FROM data
           GROUP BY month) dp ON dp.month = m.month
LEFT JOIN (SELECT MONTH(data_start) AS month, COUNT(*) AS cstart
           FROM data
           GROUP BY month) ds ON ds.month = m.month
GROUP BY m.month

输出：

month       cplan   cstart
January     0       0
February    0       0
March       1       0
April       1       0
May         0       0
June        0       0
July        3       0
August      0       1
September   2       3
October     0       2
November    0       1
December    0       0

更新的sqlfiddle

解决方案2：

WITH months(MonthNumber, Month_Name) AS
(
    SELECT 1, datename(M, '2018-01-01') Month_Name
    UNION ALL
    SELECT MonthNumber+1, datename(M, '2018-' + str(MonthNumber+1) + '-01') Month_Name
    FROM months
    WHERE MonthNumber < 12
)
select f0.Month_Name, 
(select count(*) cplan from data f1 where month(f1.data_plan)=f0.MonthNumber) cplan,
(select count(*) cstart from data f1 where month(f1.data_start)=f0.MonthNumber) cstart
from months f0

解决方案1：

WITH months(MonthNumber, Month_Name) AS
(
    SELECT 1, datename(M, '2018-01-01') Month_Name
    UNION ALL
    SELECT MonthNumber+1, datename(M, '2018-' + str(MonthNumber+1) + '-01') Month_Name
    FROM months
    WHERE MonthNumber < 12
)
select f0.Month_Name, f2.cplan, f4.cstart 
from months f0
outer apply
(
select count(*) cplan from data f1
where month(f1.data_plan)=f0.MonthNumber
) f2

outer apply
(
select count(*) cstart from data f3
where month(f3.data_start)=f0.MonthNumber
) f4

使用 COUNT() 功能：

SELECT 
    COUNT(Monthname(data_plan)) as cplan,
    COUNT(Monthname(data_start)) as cstart
FROM data
GROUP BY Monthname(data_plan)

解决方案3：

WITH months(MonthNumber, Month_Name) AS
(
    SELECT 1, datename(M, '2018-01-01') Month_Name
    UNION ALL
    SELECT MonthNumber+1, datename(M, '2018-' + str(MonthNumber+1) + '-01') Month_Name
    FROM months
    WHERE MonthNumber < 12
),
CountPlan as (
select month(data_plan) MonthNumber, count(*) cplan 
from data 
group by month(data_plan)
),
CountStart as (
select month(data_start) MonthNumber, count(*) cstart 
from data 
group by month(data_start)
)
select f0.Month_Name, 
isnull(f1.cplan, 0) cplan,
isnull(f2.cstart, 0)  cstart
from months f0
left outer join CountPlan f1 on f1.MonthNumber=f0.MonthNumber
left outer join CountStart f2 on f2.MonthNumber=f0.MonthNumber