How to have GROUP BY and COUNT include zero sums?

Question

I have SQL like this (where $ytoday is 5 days ago):

$sql = 'SELECT Count(*), created_at FROM People WHERE created_at >= "'. $ytoday .'" AND GROUP BY DATE(created_at)';

I want this to return a value for every day, so it would return 5 results in this case (5 days ago until today).

But say Count(*) is 0 for yesterday, instead of returning a zero it doesn't return any data at all for that date.

How can I change that SQLite query so it also returns data that has a count of 0?

Answer 1

Without convoluted (in my opinion) queries, your output data-set won't include dates that don't exist in your input data-set. This means that you need a data-set with the 5 days to join on to.

The simple version would be to create a table with the 5 dates, and join on that. I typically create and keep (effectively caching) a calendar table with every date I could ever need. (Such as from 1900-01-01 to 2099-12-31.)

SELECT
  calendar.calendar_date,
  Count(People.created_at)
FROM
  Calendar
LEFT JOIN
  People
    ON Calendar.calendar_date = People.created_at
WHERE
  Calendar.calendar_date >= '2012-05-01'
GROUP BY
  Calendar.calendar_date