left join, inner join, one before the other, other issue? - mysql

Question

I have a form and MySQL that works with this:
$sql = "
SELECT p.*
, t.*, f.name AS fname
FROM table_posts p
JOIN table_topics t
USING(tid)
JOIN table_forums f
ON f.fid = t.fid
WHERE f.fid IN($forums)";
I wish to add an option on the form so that any tid's in the subscriptions table for the user are excluded from the results from the sql above:
$sql .= " LEFT JOIN (SELECT x.tid FROM table_subscriptions AS x WHERE x.username = '$user' AND x.type = 'x' GROUP BY x.tid) AS query2 USING (tid) AND query2.tid IS NULL";
however, am encountering
MySQL encountered the following error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'LEFT JOIN (
Is the issue because the left join should be done first, or is there some other issue? Any clues for corrections appreciated.

Answer 1

You can bring in the subscriptions table with a LEFT JOIN, and then filter on unmatched records in the WHERE clause
SELECT
p.*,
t.*,
f.name AS fname
FROM table_posts AS p
INNER JOIN table_topics AS t USING (tid)
INNER JOIN table_forums AS f ON f.fid = t.fid
LEFT JOIN table_subscriptions AS s ON s.username = ? AND s.type = 'x' AND s.tid = p.tid
WHERE
f.fid IN (?, ?, ?)
AND s.tid IS NULL
Note: I updated your query so it uses query parameters instead of concatenating variables in the SQL string: this is both more efficient and secure.
You can also use NOT EXISTS:
SELECT
p.*,
t.*,
f.name AS fname "
FROM table_posts AS p
INNER JOIN table_topics AS t USING (tid)
INNER JOIN table_forums AS f ON f.fid = t.fid
WHERE NOT EXISTS (
SELECT 1
FROM table_subscriptions AS s
WHERE s.username = ? AND s.type = 'x' AND s.tid = p.tid
)
WHERE f.fid IN (?, ?, ?)