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.