many to many table join mysql

Question

i have three tables:

mails

+-----+----------+----------------+
| id  | chain_id | subject        |
+-----+----------+----------------+
| 108 |      108 | aaa            |
| 109 |      109 | Chain          |
| 110 |      110 | New            |
| 111 |      108 | aaa            |
| 112 |      108 | Re: Re:  aaa   |
| 113 |      109 | Chain          |
| 114 |      114 | Fwd: Subject   |
| 115 |      108 | Fwd: aaa       |
| 116 |      108 | Re: aaa        |
| 117 |      108 | Fwd: aaa       |
+-----+----------+----------------+

chain_tags

+----+----------+--------+
| id | chain_id | tag_id |
+----+----------+--------+
|  1 |      108 |     15 |
|  2 |      108 |     16 |
+----+----------+--------+

mail_tags

+----+--------+---------+
| id | name   | color   |
+----+--------+---------+
| 15 | First  | #ffdaef |
| 16 | Second | #aed9ff |
+----+--------+---------+

I want to select all records from mails table, mail_tags.color , mail_tags.name , and the quantity of records with the same chain_id , group by mails.chain_id , but after group by i want to leave the latest records (with the biggest id).

I managed to write this query

SELECT
        *,
        COUNT(*) AS quantity
    FROM
        (SELECT
            *
        FROM
            mails
        ORDER BY id DESC) AS t
    GROUP BY chain_id
    ORDER BY id DESC

it returns last rows group by chain_id with total quantity of mails in that chain, but i don't know how to join tag names and tag colors for each record. Some of the records might have few tags, some none.

Answer 1

I'm not too sure if I understood the exact requirement but something along these lines should work:

select x.id, x.chain_id, x.subject, count, mt.name, mt.color from
(
select m.*,count(*) count,max(mt.id) maxid
   from mails m
left outer join chain_tags ct on (m.chain_id=ct.chain_id)
left outer join mail_tags mt on (ct.tag_id=mt.id)
  group by m.chain_id
) x
left join mail_tags mt on x.maxid=mt.id

basically the idea is to obtain the count and the max mail_tag id by chain_id, which is done within the inner query and once we know the max mail tag id we can join back to mail_tags and get the name and color, which is done by the left join in the main query. Hope this helps