Question

I have 3 tables, message, subject and message_subject_rel. The idea is to have messages that can relate to a lot of subjects and then do a cross subject search.

Lets say I have a message:

Id: 1, Message: This is a message 

2 subjects:

Id:1, Subject: Math Id:2, Subject: Science 

And there’s 2 message_subject_rel entries that go:

Id: 1, message_id: 1, subject_id: 1 Id: 2, message_id: 1, subject_id: 2 

If i wanted to search the messages that are related with math, I would do a simple join with the 3 tables and the where clause would be subject = ‘Math’

But what I don’t know how to do, is search for the messages that are related with math AND Science. If i do a simple join i get tables with something like:

id  message     user_id     created_at  ip  id  message_id  subject_id  id  subject 

And if I do a where subject = 'Math' and subject = 'Science' i wont get any results because each message will only have 1 subject related in each row, but duplicated rows for messages with more than 1 subject.

So, what do you recommend?

Answer 1

This is basically the same question as Filtering from join-table

I’ll adapt my answer from that question.

JOIN solution:

SELECT m.* FROM messages m  JOIN message_subject_rel ms1 ON (m.id = ms1.message_id)  JOIN subjects s1 ON (ms1.subject_id = s1.id AND s1.subject = 'Math')  JOIN message_subject_rel ms2 ON (m.id = ms1.message_id)  JOIN subjects s2 ON (ms2.subject_id = s2.id AND s2.subject = 'Science'); 

GROUP BY solution:

Note that you need to list all m.* columns in the GROUP BY clause, unless you use MySQL.

SELECT m.* FROM messages m   JOIN message_subject_rel ms ON (m.id = ms.message_id)  JOIN subjects s ON (ms.subject_id = s.id) WHERE s.subject IN ('Math', 'Science')) GROUP BY m.id, ... HAVING COUNT(*) = 2; 

Subquery solution:

SELECT m.* FROM messages m WHERE m.id = ANY (SELECT message_id                    FROM message_subject_rel ms JOIN subjects s                      ON (ms.subject_id = s.id)                    WHERE s.subject = 'Math')   AND m.id = ANY (SELECT message_id                    FROM message_subject_rel ms JOIN subjects s                      ON (ms.subject_id = s.id)                    WHERE s.subject = 'Science'); 

Modified GROUP BY solution:

Simplifies GROUP BY clause by isolating search in a subquery.

SELECT m.* FROM messages m WHERE m.id IN (   SELECT ms.message_id FROM message_subject_rel ms JOIN subjects s     ON (ms.subject_id = s.id)   WHERE s.subject IN ('Math', 'Science'))   GROUP BY ms.message_id HAVING COUNT(*) = 2 ); 

PS: There’s no reason your message_subject_rel table needs an ID column.