Using group by with left join and IN yields empty result - sql

Question

I have two tables: invitees and invitee_information. invitees belong to events (third table event_id is a foreign key in invitees table).
I want to get the total count of invitees per event for which there is no information available yet (no record in the invitee_information table).
When I write the query using only one event_id in the where clause and anti-join pattern (IS NULL for invitee_id column to filter for rows having no information saved) the count is returned as expected for that given event_id:
SELECT COUNT(i.id)
FROM invitees i
LEFT JOIN invitee_information ii
ON ii.invitee_id = i.id
WHERE ii.invitee_id IS NULL AND i.`event_id` = 18571 AND i.`invitation_sent` = 1;
Now in order to optimize the query to get this count data at DB level for multiple event_ids instead of getting this in a loop for each event_id, I used IN and passed multiple event_ids and used GROUP_BY event_id.
Expected result should be (if there is no data saved in invitee_information table for event_id 18569 :
event_id | count(i.id)
18569 | 0
But query result is always empty. My updated query is:
SELECT i.`event_id`, COUNT(i.id)
FROM invitees i
LEFT JOIN invitee_information ii
ON i.id = ii.invitee_id
WHERE ii.invitee_id IS NULL AND i.`event_id` IN(18569,18571) AND i.`invitation_sent` = 1
GROUP BY i.`event_id`;

Answer 1

The only way I could imagine this happening is if you have internationalization settings such that , means a decimal point. You can easily test this using:
SELECT i.`event_id`, COUNT(i.id)
FROM invitees i LEFT JOIN
invitee_information ii
ON i.id = ii.invitee_id
WHERE ii.invitee_id IS NULL AND
(i.`event_id` = 18569 OR i.`event_id` = 18571) AND
i.`invitation_sent` = 1
GROUP BY i.`event_id`;
If this works, then at least we have identified the problem. I think a space then works:
SELECT i.`event_id`, COUNT(i.id)
FROM invitees i LEFT JOIN
invitee_information ii
ON i.id = ii.invitee_id
WHERE ii.invitee_id IS NULL AND
i.`event_id` IN (18569, 18571) AND
i.`invitation_sent` = 1
GROUP BY i.`event_id`;
I should note that you probably want COUNT(ii.invitee_id) in the SELECT rather than COUNT(i.id). The latter will never return 0.
EDIT:
Perhaps the two events are not in the invitees table. Assuming you have an events table, then something like this:
SELECT e.event_id, COUNT(i.id)
FROM events e LEFT JOIN
invitees i
ON i.event_id = e.event_id AND
i.invitation_sent = 1 LEFT JOIN
invitee_information ii
ON i.id = ii.invitee_id
WHERE ii.invitee_id IS NULL AND
e.event_id IN (18569, 18571) AND
GROUP BY e.event_id;