Show all dates even those with zero data in it in mySQL? - php

Question

My following query shows the date and the count of the emails found on each day (last 2 days)
My problem is that if no emails are found today, the today date will not be displayed on the output. (if yesterday has emails, it will show only 1 row with yesterday date and email).
How can I edit my query to always show 2 rows, today and yesterday, date and number of emails even zero?
SELECT maildate,
COUNT(*) AS totalEmails
FROM emails
WHERE maildate >= Date_add(Curdate(), interval - 2 DAY)
AND company_id = 1
GROUP BY DATE(maildate)
ORDER BY maildate desc

Answer 1

There are many tricks to creating a list of dates (or numeric sequences similarly). The one I like to use with MySQL is using #sqlvariables. I will typically start with a baseline value such as your date -2 days. I will do a cross-join to any other table in the database that has at least as many records as you expect in your output... Say 30 days, or a whole year 366 days, or longer. The inner sql variable prep will keep increasing itself by whatever increment (you could even do date ranges such as begin/end of a week, month, etc). Now you have your table of all possible dates you are looking to fill.
Now, I do a secondary query by the value -- in this case your email date and apply the group by. Using the where clause in this query will make IT faster since it can utilize the date on its query result set before returning for the LEFT-JOIN to the date range result set.
Now, your simple left-join gets both parts of all dates to be included and those corresponding counts that do exist.
Note the table alias "AnyTableWithAtLeast3RecordInIt" in the "JustDates" query could in-fact be your "emails" table. Since we don't care about any criteria except a record exists, and we are applying a limit of 30 days in my example, it will be instantaneous.
select
JustDates.DateToInclude,
coalesce( SumCnts.TotalEmails, 0 ) TotalEmails
from
( select
#myDate := DATE_ADD( #myDate, INTERVAL 1 DAY ) as DateToInclude
from
( select #myDate := Date_add(Curdate(), interval - 2 DAY) ) as SQLVars,
AnyTableWithAtLeast3RecordInIt
limit 30 ) JustDates
left join
( select
maildate,
COUNT(*) AS totalEmails
FROM
emails
WHERE
maildate >= Date_add(Curdate(), interval - 2 DAY)
AND company_id = 1
GROUP BY
DATE(maildate) ) SumCnts
ON JustDates.DateToInclude = SumCnts.MailDate
Now, judging by your query, but unclarified request... Your emails table CAN HAVE FUTURE DATES? Is that correct? Such as a Dr. Office and appointments are for the future and you want to get emails out for a given range. This is what I was inferring and hence had my limit to only go out 30 days... If you need longer, just extend the LIMIT clause.

Answer 2

You need a table that contains all dates in the needed range. If its only about today and yesterday, you can easily create it as a subquery (derived table).
SELECT Curdate() as maildate
UNION ALL
SELECT Curdate() - INTERVAL 1 DAY
http://rextester.com/ALH50651
Now you can LEFT JOIN your table and count the rows:
SELECT sub.maildate,
COUNT(m.maildate) AS totalEmails
FROM (
SELECT Curdate() as maildate
UNION ALL
SELECT Curdate() - INTERVAL 1 DAY
) sub
LEFT JOIN emails m
ON DATE(m.maildate) = sub.maildate
AND m.company_id = 1
GROUP BY sub.maildate
ORDER BY sub.maildate desc