Question

Not sure if my title explains it very well so here’s an example. I have two MYSQL tables, comments and votes_comments which are matched based on a field comment_id. For sample data comments has two rows and votes_comments has one row which matches one of the comments row. AKA Two comments were written and a single person voted on one of them Given that, the following query returns 2 rows:

SELECT comments.*, users.username FROM comments 
JOIN users ON users.user_id = comments.user_id 
WHERE comments.track_id=6 AND comments.parent_id=-1
GROUP BY comments.comment_id

Now what I want to do is join on votes_comments which may or may not have a matching row. What I want to do is grab the matching value if it exists or return null in the column if there is no matching result. AKA: Did the user vote it up, down or not at all (null). Here is my modified query:

SELECT comments.*, users.username, votes_comments.vote FROM comments 
JOIN users ON users.user_id = comments.user_id 
LEFT JOIN votes_comments ON votes_comments.comment_id = comments.comment_id
WHERE track_id=6 AND parent_id=-1 AND votes_comments.user_id=1
GROUP BY comments.comment_id

This query only returns one row since I only have a single row in votes_comments. I thought that left join would return results the way I want but apparently not. Any ideas how to get what I’m looking for?

Answer 1

I suspect the WHERE clause AND votes_comments.user_id=1 is causing the problem.

Read a tutorial on Joins and you’ll see that the result of your LEFT JOIN is a temporary table with two rows, and for each row, fields from comments JOIN users and from votes_comments. If no matching record from votes_comments exists, the fields in the temporary table are filled with NULL values.

Your WHERE clause, AND votes_comments.user_id=1, will fail because votes_comments.user_id is NULL in the rows of user with no matching votes_comments record.

If what you actually want is to join to the row in votes_comments what matches a specific user ID (e.g. ID=1), then your ON clause should be something like

ON votes_comments.comment_id = comments.comment_id
    AND votes_comments.user_id=1