Question

Hows is it possible for these two queries to be different. I mean the first query didn’t include all the rows from my left table so I put the conditions within the join part.

Query 1

SELECT COUNT(*) as opens, hours.hour as point 
FROM hours 
LEFT OUTER JOIN tracking ON hours.hour = HOUR(FROM_UNIXTIME(tracking.open_date)) 
WHERE tracking.campaign_id = 83 
AND tracking.open_date < 1299538799 
AND tracking.open_date > 1299452401 
GROUP BY hours.hour

Query 2

SELECT COUNT(*) as opens, hours.hour as point 
FROM hours 
LEFT JOIN tracking ON hours.hour = HOUR(FROM_UNIXTIME(tracking.open_date)) 
AND tracking.campaign_id = 83 
AND tracking.open_date < 1299538799 
AND tracking.open_date > 1299452401 
GROUP BY hours.hour

The difference is that the first query gives me 18 rows where there are no rows between point 17 to 22. But when I run the second query, it shows the fully 24 rows but for rows between 17 and 22 it has a value of 1! I would of expected it to be 0 or NULL? If it really is 1 should it not have appeared in the first query?

How has this happened?

Answer 1

You’re using COUNT(*), which will count every row in your result set (as it’s written), since even without data in tracking, you do have data in hours.

Try changing COUNT(*) to COUNT(tracking.open_date) (or any non-nullable column within tracking; it doesn’t matter which one).