How to use multiple IN - Where IN (...) and IN (...) - mysql

Question

I'm tuning performance of my script and I changed the way posts categories are sorted.
I was using successfully the following query.
It selects posts with (130|3|4|5) categories and checks if it also has (73) category init as one post can have multiple categories.
SELECT *
FROM post
LEFT JOIN post_plus
ON ( post.id = post_plus.news_id )
WHERE category REGEXP '[[:<:]](130|3|4|5)[[:>:]]'
AND category REGEXP '[[:<:]](73)[[:>:]]'
AND approve = 1
ORDER BY fixed DESC,
date DESC
LIMIT 0, 7
Now with a new query I need to perform multiple IN (...) queries instead of AND category REGEXP '...', but for some reason additional AND categoryid IN always returns empty result.
SELECT *
FROM post
LEFT JOIN post_plus
ON ( post.id = post_plus.news_id )
LEFT JOIN post_category
ON ( post_category.postid = post.id )
WHERE categoryid IN ( 130, 3, 4, 5 )
AND categoryid IN ( 73 )
AND approve = 1
ORDER BY fixed DESC,
date DESC
LIMIT 0, 7;
post_category structure
+-----+--------+------------+
| cid | postId | categoryId |
+-----+--------+------------+
| 824 | 7 | 10 |
| 825 | 7 | 13 |
| 826 | 7 | 16 |
| 827 | 7 | 29 |
| 828 | 7 | 71 |
+-----+--------+------------+

Answer 1

To get the postid for posts that are both in category 73 and in at least one of category id 130, 3, 4, 5 you can use
SELECT postid
FROM post_category
GROUP BY postid
HAVING MAX(CASE
WHEN categoryid = 73 THEN 1
END) = 1
AND MAX(CASE
WHEN categoryid IN ( 130, 3, 4, 5 ) THEN 1
END) = 1
This can be used in a derived table to join onto your wider query.
Or another possibility is
SELECT c1.postid
FROM post_category c1
JOIN post_category c2
ON c1.postid = c2.postid
WHERE c1.categoryid IN ( 130, 3, 4, 5 )
AND c2.categoryid = 73

Answer 2

You can use an INTERSECT-compound statement:
SELECT postid
FROM post_category
WHERE categoryid = 73
INTERSECT
SELECT postid
FROM post_category
WHERE categoryid IN (130, 3, 4, 5)
This will select only those postids that have a categoryid equal to 73 and at least one of categoryid = 130, 3, 4, or 5.
This query will probably run faster than a aggregate-query (GROUP BY), since the sql-server can use the index on categoryid (which you hopefully have) to get the relevent postids and then uses a temporary index to compute the intersection (at least thats what sqlite does). If there there are no postids with categoryid 73, then the second part of the query is never executed, since the intersection of an empty set with anything else is always just the empty set.
To get your posts, use a subquery:
SELECT post.*
FROM post
WHERE post.id IN (SELECT postid
FROM post_category
WHERE categoryid = 73
INTERSECT
SELECT postid
FROM post_category
WHERE categoryid IN (130, 3, 4, 5))
Also note that you can add more INTERSECT-compounds as you like.