Question

I have two sql queries, one which retrieves all the Event records for a given topic id and another which is supposed to retrieve all Event records excluding given topic id, but doesn’t.

The first query retrieves the correct records

SELECT `events`.* 
FROM `events` INNER JOIN events_topics ON events.id = events_topics.event_id
WHERE (events_topics.topic_id = 75)

The second query which is supposed to exclude events does not exclude any.

SELECT `events`.* 
FROM `events` INNER JOIN events_topics ON events.id = events_topics.event_id 
WHERE (events_topics.topic_id <> 75)

Answer 1

What this means is that you have other topic_id values in events_topic for a given event. A.k.a., if you have topic_id = 75 and topic_id = 33 for, say, eventid = 45, this eventid will be returned because the query matches on topic_id = 33

You have asked:

“give me all events where topic_id <> 75 does not exist”

Change the query to EXISTS/NOT EXISTS which is correct. This is:

“give me all events where topic_id = 75 does not exist”

SELECT `events`.* 
FROM `events`
WHERE
    EXISTS (SELECT * FROM events_topics.event_id
            WHERE events_topics.topic_id = 75
                     AND
                      events.id = events_topics.event_id
            )


SELECT `events`.* 
FROM `events`
WHERE
    NOT EXISTS (SELECT * FROM events_topics.event_id
            WHERE events_topics.topic_id = 75
                     AND
                      events.id = events_topics.event_id
            )