Question

Following a previous question ( How to do this query in Mysql? )

Lets say I have a message:

Id: 1, Message: This is a message 

2 subjects:

Id:1, Subject: Math Id:2, Subject: Science Id:3, Subject: Numbers 

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 Id: 3, message_id: 1, subject_id: 3 

I wanted to do a query to select the messages that had Math AND Science as subjects I ended up using:

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'); 

Now, its very clear that i would like to show the message(because it does have those 2 subjects) and also tell the user that it doesn’t ONLY have those 2 subject, but it actually have 3, and not only that… i would like to print the 3 subjects(Of course i have lots of messages and would like to actually list them, with their respective subjects in just 1 query). I cant seem to understand if that is actually possible with just one query, since i would get n ‘subject’ field as a return.

Any idea if this can be done with just one query and if so, how?

TY

Answer 1

See GROUP_CONCAT for one solution. I don’t know how efficient this approach is, however.

select message.Id as id, Message, group_concat(Subject separator ';') as subjects from message,subject,message_subject_rel where message.Id=message_subject_rel.message_id   and subject.Id=message_subject_rel.subject_id group by message.Id having subjects like '%Math%' and subjects like '%Science%'; 

From that, I get:

+------+---------+----------------------+ | id   | Message | subjects             | +------+---------+----------------------+ |    1 | Message | Math;Science;Numbers |  +------+---------+----------------------+ 

Obviously, you could do a better job than my ‘;’ separator and my like clauses.