Sort data from MYSQL database by hour - mysql

Question

I have users in db that I want to sort by hour and display count of users registered at that hour.
select
date_format(create_time, '%Y-%m-%d %h%p') as date,
count(id) as 'Number of registrations'
from users
group by 1
order by 1 desc
;
The above code will work; however, what I am trying to do is display 0's for the hours that have no user registrations. For example, if there were no registrations at 5pm, this will skip row for 5pm, which is logical. Is there a way to achieve what I am trying?

Answer 1

You could use a query like this:
select
date_format(t.d + INTERVAL t.h HOUR, '%Y-%m-%d %h%p') as date,
count(id) as 'Number of registrations'
from (
SELECT *
FROM
(SELECT DISTINCT DATE(create_time) d FROM users) dates,
(SELECT 0 h UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3
UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7
UNION ALL SELECT 8 UNION ALL SELECT 9 UNION ALL SELECT 10 UNION ALL SELECT 11
UNION ALL SELECT 12 UNION ALL SELECT 13 UNION ALL SELECT 14 UNION ALL SELECT 15
UNION ALL SELECT 16 UNION ALL SELECT 17 UNION ALL SELECT 18 UNION ALL SELECT 19
UNION ALL SELECT 20 UNION ALL SELECT 21 UNION ALL SELECT 22 UNION ALL SELECT 23) hours
) t LEFT JOIN users
ON DATE(users.create_time)=t.d AND HOUR(users.create_time)=t.h
group by t.d, t.h
order by t.d, t.h
Please see fiddle here.

Answer 2

You need to generate all possible day and hour combinations.
Assuming that you have at least one record on each day and one record for each hour, you can do:
select concat(d.theday, ' ', h.thehour) as date,
count(id) as 'Number of registrations'
from (select distinct date_format(create_time, '%Y-%m-%d') as theday from users
) d cross join
(select distinct date_format(create_time, '%h%p') as thehour from users
) h left outer join
users u
on date_format(u.create_time, '%Y-%m-%d %h%p) = concat(d.theday, ' ', h.thehour)
group by concat(d.theday, ' ', h.thehour)
order by 1 desc;