MySQL Join; 仅查询不匹配的表字段

Question

Hopefully, I can explain this clearly, but, for about two days now, I have been trying to select data from one table that does not exist in the other table.

So, basically, I want to select from a list of tracks that have not already been listened to .

SELECT *
FROM tracks JOIN listens
ON listens.user_id = tracks.user_id
WHERE listens.user_id != 9;

PS: I have tried many solutions including unions and subqueries. This is where I got to before giving up on figuring it out myself.

My table setup:

[tracks] ID|user_id

[LISTENS] ID|track_id|user_id

I want only the tracks that have not been listened to, and, for some reason I cannot achieve this. It just returns all of the tracks, or even sometimes discludes tracks not yet listened to.

Any points on the correct approach ? Would be greatly appreciated! Thanks much :D

Answer 1

To find tracks that user 9 has not listed to, here is one method:

SELECT t.*
FROM tracks t left join
     listens l
     on l.track_id = t.id AND l.user_id = 9
WHERE l.user_id is null;