MySQL 5.7

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create table transactions
	(`id` int, `number` varchar(3), `created_at` datetime)
;
	
insert into transactions
	(`id`, `number`, `created_at`)
values
	(1, 'a11', '2019-01-01 06:08:19'),
	(2, 'a21', '2019-01-04 06:08:19')
;

Records: 2  Duplicates: 0  Warnings: 0


create table sequence(id int);
create procedure insert_data_proc(in v_i int)
begin
declare i int default 0;
while i < v_i
do
insert into sequence values (i);
set i = i + 1;
end while;
end;
call insert_data_proc(10);
drop procedure insert_data_proc;
select 
	t1.created_at the_date
	,case when count is null then 0 else count end as count 
from (
	select date_add(t2.mincreated_at , interval id day) created_at
	from sequence t1
	left join (
	   select datediff(max(created_at),min(created_at)) diff
	      ,date(min(created_at) ) mincreated_at
	      ,date(max(created_at) ) maxcreated_at
	   from transactions
	) t2 on 1=1
	where t1.id < t2.diff+1
) t1
left join (
	select date(created_at) created_at,count(1) count
	from transactions
	group by date(created_at)
) t2 on t1.created_at = t2.created_at
order by the_date


the_date
count


2019-01-01
1

2019-01-02
0

2019-01-03
0

2019-01-04
1