Left Outer Join doesn't return all rows from my left table?

I am trying to get the number of page opens on a per day basis using the following query.

SELECT day.days, COUNT(*) as opens 
FROM day 
LEFT OUTER JOIN tracking ON day.days = DAY(FROM_UNIXTIME(open_date)) 
WHERE tracking.open_id = 10 
GROUP BY day.days

The output I get it is this:

days opens
1   9
9   2

The thing is, in my day table, I have a single column that contains the number 1 to 30 to represent the days in a month. I did a left outer join and I am expecting to have all days show on the days column!

But my query is doing that, why might that be?

Nanne's answer given explains why you don't get the desired result (your WHERE clause removes rows), but not how to fix it.

The solution is to change WHERE to AND so that the condition is part of the join condition, not a filter applied after the join:

SELECT day.days, COUNT(*) as opens 
FROM day 
LEFT OUTER JOIN tracking
ON day.days = DAY(FROM_UNIXTIME(open_date)) 
AND tracking.open_id = 10 
GROUP BY day.days

Now all rows in the left table will be present in the result.

You specify that the connected tracking.open_id must be 10. For the other rows it will be NULL, so they'll not show up!

The condition is in the WHERE clause. After joining the tables the WHERE conditions are evaluated to filter out everything matching the criteria.Thus anything not matching tracking.open_id = 10 gets discarded.

If you want to apply this condition while joining the two tables, a better way is to use it with the ON clause (i.e. joining condition) than the entire dataset condition.