MySQL 5.7

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create table test(date_value date, value int);
insert into test values
('2020-01-01',1),
('2020-01-01',2),
('2020-01-03',2),
('2020-01-07',3),
('2020-01-08',4),
('2020-01-08',1);

Records: 6  Duplicates: 0  Warnings: 0


SELECT
   date_table.date,
   ifnull(SUM(value),0) as sum_val
from (
select DATE_ADD('2020-01-01', INTERVAL (@i:=@i+1)-1 DAY) as `date`
from information_schema.columns,(SELECT @i:=0) gen_sub
where DATE_ADD('2020-01-01',INTERVAL @i DAY) BETWEEN '2020-01-01' AND '2020-01-08'
) date_table
left join test on test.date_value = date_table.date
GROUP BY date;


date
sum_val


2020-01-01
3

2020-01-02
0

2020-01-03
2

2020-01-04
0

2020-01-05
0

2020-01-06
0

2020-01-07
3

2020-01-08
5

date	sum_val
2020-01-01	3
2020-01-02	0
2020-01-03	2
2020-01-04	0
2020-01-05	0
2020-01-06	0
2020-01-07	3
2020-01-08	5