Question

Below is a simple query for a click tracker. I’ve had a look at a lot of other posts and I’m scratching my head. I cannot get this query to work so that all rows from the calendar table (one calendar day per row) are displayed:

SELECT DATE_FORMAT(calendar_date, '%a %D') AS calendar_date, 
count( tracker_id ) as clicks
FROM calendar
LEFT JOIN offer_tracker USING(calendar_id)
WHERE 
  calendar_month = Month(CURDATE()) AND 
  calendar_year = Year(CURDATE()) AND ( offer_id = 4 OR offer_id IS NULL )
GROUP BY calendar_date;

It’s nearly there but not all rows in the calendar table are returned i.e. there is no Fri 2nd, Tue 6th, Wed 7th etc:

Does anyone have any ideas on where I’m going wrong? Should I be using a subquery?

Answer 1

I guess the offer_id is from the offer_tracker table. When you have an (left) outer join, and you use a field from the right table in a WHERE condition (like your offer_id = 4), the join is actually cancelled and gives same results as an inner join.

The attempt to lift the cancellation (offer_id = 4 OR offer_id IS NULL) does not work as you expect. Any row from offer_tracker with offer_id <> 4 has already passed the LEFT JOIN but is removed because of the WHERE condition. So, no row with Friday 2nd will appear in the results if there is a row with offer_id different than 4 for this date.

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Move the offer_id = 4 check to the LEFT JOIN, instead:

SELECT DATE_FORMAT(calendar_date, '%a %D') AS calendar_date
     , count( tracker_id ) as clicks
FROM calendar
  LEFT JOIN offer_tracker 
    ON  offer_tracker.calendar_id = calendar.calendar_id
    AND offer_id = 4
WHERE calendar_month = Month(CURDATE())
  AND calendar_year = Year(CURDATE()) 
GROUP BY calendar_date