How can I use IN() clause in the INNER JOIN? - mysql

Question

Here is my query:
SELECT p.*
FROM posts p
INNER JOIN pivot pi on p.id = pi.post_id
INNER JOIN tags t on t.id = pi.tag_id and t.name = "mysql"
All I'm trying to do is getting posts that are tagged with a specific tag. In query above, I'm selecting all posts with mysql tag.
Now I need to make it working for multiple-tags. I mean, I want to select all post which are tagged with both mysql and php. I guess I have to use IN() clause in the query. But I don't know how exactly. Any idea?
EX: Here are my tables:
// posts
+----+---------+------------+
| id | subject | body |
+----+---------+------------+
| 1 | sub1 | content1 |
| 2 | sub2 | content2 |
+----+---------+------------+
// tags
+----+--------+
| id | name |
+----+--------+
| 1 | mysql |
| 2 | php |
+----+--------+
// pivot
+---------+--------+
| post_id | tag_id |
+---------+--------+
| 1 | 1 |
| 2 | 1 |
| 2 | 2 |
+---------+--------+
The expected result for [mysql]:
| 1 | sub1 | content1 |
| 2 | sub2 | content2 |
The expected result for [mysql][php]:
| 2 | sub2 | content2 |

Answer 1

I would use a WHERE clause for the conditions and then aggregation:
SELECT p.*
FROM posts p INNER JOIN
pivot pi
ON p.id = pi.post_id INNER JOIN
tags t
ON t.id = pi.tag_id
WHERE t.name IN ('mysql', 'php')
GROUP BY p.id
HAVING COUNT(*) = 2; -- this is the number of tags in the list
This assumes that tags are not duplicated.
A note about using GROUP BY p.id with SELECT p.*. This is actually valid and allowed by the ANSI standard, assuming that p.id is unique in posts. The specific rule is based on functional dependency.
I do think the most recent versions of MySQL disallow the construct by default. You can always do something like:
SELECT p.*
FROM posts p JOIN
(SELECT pi.post_id
FROM pivot pi INNER JOIN
tags t
ON t.id = pi.tag_id
WHERE t.name IN ('mysql', 'php')
GROUP BY p.id
HAVING COUNT(*) = 2 -- this is the number of tags in the list
) t
ON p.id = t.post_id ;

Answer 2

I want to select all post which are tagged with both mysql and php. I guess I have to use IN() clause in the query.
Nope. That would show you posts which are tagged one or the other. To find posts with both, I would join to the tables twice:
SELECT p.*
FROM posts p
INNER JOIN pivot pi1 on p.id = pi1.post_id
INNER JOIN tags t1 on t1.id = pi1.tag_id and t1.name = "mysql"
INNER JOIN pivot pi2 on p.id = pi2.post_id
INNER JOIN tags t2 on t2.id = pi2.tag_id and t2.name = "php"
...

Answer 3

I don't know why should we complicate this ..
Instade of in clause just use string input to query and compare it with string aggregate.
SELECT *
FROM posts P
INNER JOIN (
SELECT post_id, CONCAT ("(", Group_concat(t.name SEPARATOR ' ,'), ")") AS tgs
FROM pivote p
INNER JOIN tag t
ON (t.id = p.tag_id)
GROUP BY post_id
) AA
ON P.id = post_id
WHERE tgs LIKE "(mysql,php)