Predefined range of date to fill

Question

I want to count the number of item by date, but sometimes the table doesn't contains rows with some dates, and I have a hole in the result, like :

+------------+--------+
| date       | number |
+------------+--------+
| 2021-03-12 |      1 |
| 2021-03-15 |      1 |
| 2021-03-17 |      1 |
| 2021-03-18 |      1 |
+------------+--------+

I would like this result:

+------------+--------+
| date       | number |
+------------+--------+
| 2021-03-12 |      1 |
| 2021-03-13 |      0 |
| 2021-03-14 |      0 |
| 2021-03-15 |      1 |
| 2021-03-16 |      0 |
| 2021-03-17 |      1 |
| 2021-03-18 |      1 |
+------------+--------+

So, I created a temporary table with WITH, it works:

WITH recursive Date_Ranges AS (
       select DATE(NOW() - INTERVAL 3 MONTH) AS Date
       union
       select Date + interval 1 day
       from Date_Ranges
       where Date < DATE(NOW())
   )
    SELECT *, 0 AS number FROM Date_Ranges

Next, I want to retrieve the statistics, I tried with this query:

WITH recursive Date_Ranges AS (
   select DATE(NOW()- INTERVAL 3 MONTH) AS Date
   union
   select Date + interval 1 day
   from Date_Ranges
   where Date < DATE(NOW())
)
select *, '0' AS value from Date_Ranges
UNION DISTINCT 
SELECT DATE(time) AS date, count(*) AS number
                    FROM my_table
                    WHERE time >= NOW()- INTERVAL 3 MONTH
                    AND time <= DATE(NOW())
                    GROUP BY DATE(ca_time)

But the UNION doesn't work because the value is different so mysql keep the 2 lines:

example:
+------------+--------+
| date       | number |
+------------+--------+
| 2021-03-12 |      0 |
| 2021-03-13 |      0 |
| 2021-03-14 |      0 |
| 2021-03-15 |      0 |
| 2021-03-16 |      0 |
| 2021-03-17 |      0 |
| 2021-03-18 |      0 |
| 2021-03-12 |      2 |
| 2021-03-13 |      6 |
| 2021-03-14 |      7 |
| 2021-03-15 |      8 |
| 2021-03-16 |      9 |
| 2021-03-17 |      5 |
| 2021-03-18 |      0 |
+------------+--------+

How the lines with 0 can be merge with the second union ?

Thanks.

Edit:

I tried with this query, but dates without result are not here:

WITH recursive dates AS (
   SELECT DATE(NOW() - INTERVAL 3 MONTH) AS Date
   UNION ALL
   SELECT Date + interval 1 day
   FROM dates
   WHERE Date < DATE(NOW())
)
SELECT date, COALESCE(COUNT(1), 0) AS number
FROM dates
LEFT JOIN geo
ON dates.date = DATE(geo.ca_time)
WHERE departmentCode = '89'
GROUP BY geo.ca_time
ORDER BY geo.ca_time

result:

+------------+--------+
| date       | number |
+------------+--------+
| 2021-02-21 |      1 |
| 2021-02-22 |     11 |
| 2021-02-27 |      8 |
...

Answer 1

UNION ALL version

WITH recursive Date_Ranges AS (
   select DATE(NOW()- INTERVAL 3 MONTH) AS Date
   union
   select Date + interval 1 day
   from Date_Ranges
   where Date < DATE(NOW())
)   
select date, count(*) - 1 AS number
from (
   select date
   from Date_Ranges
   union all
   select DATE(time) 
   from my_table
   where time >= NOW() - INTERVAL 3 MONTH
             AND time <= DATE(NOW())
) t
group by date

db<>fiddle

Answer 2

join your cte with your table :

WITH recursive Date_Ranges AS (
   select DATE(NOW()- INTERVAL 3 MONTH) AS Date
   union
   select Date + interval 1 day
   from Date_Ranges
   where Date < DATE(NOW())
)

select dr.date ,coalesce(sum(t.num),0)
from Date_Ranges dr
left join my_table t
 on dr.date = t.datecol
group by dr.date

db<>fiddle here