Postgres 11

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create table views(id bigint, created_at timestamp);

create table items(id bigint, created_at timestamp);

insert into views(id, created_at) values

('1', '2018-12-28 22:46:35'),

('2', '2018-12-28 22:46:35'),

('3', '2018-12-28 22:46:35'),

('4', '2018-12-28 22:46:35'),

('5', '2018-12-28 22:46:35');

insert into items(id, created_at) values

('1', '2018-12-28 22:46:35');

5 rows affected

1 rows affected

select

dates.d as day,

count(v.*) as views_count,

count(i.*) as items_count

from (

select d from generate_series('2018-12-01'::date, '2018-12-30', '1 day' ) as d

) as dates

left join views as v on v.created_at::date = dates.d

left join items as i on i.created_at::date = dates.d

group by day order by day desc;