SQL Sum sub-query with joined tables?

Question

I have a forum site with posts (gems) and file attachments (gemdetail) along with replies (gems) to the posts and the replies can also have file attachments (gemdetail). Since both posts and replies are stored in the same table, it makes for an interesting left join which selects all posts with associated replies and detail.

I want to add another table to the mix (rating) which allows the user to rate each post. I then want to be able in the same query to get the sum total rating for each post. How to add sum(rating) so each row of the output will have the sum for the gemid. I know I need a sum sub-query (derived table w/ ad-hoc result sets) similar to the one found here , but it is above my skill set. Thanks in advance.

The table structure is as follows

table: gems
gemid    title        replygemid
-----    -----        ----------
220      map              NULL
223      inhabitants      NULL
403      reply to map     220

table: gemdetail
gemid    filename
------   --------
220      uganda-map.jpg
220      mozambique-map.jpg
223      uganda-inhabitants.jpg
223      kenya-inhabitants.jpg
403      mona-lisa-x8.jpg 

table: rating (to be added)
gemid    rating
-----    -------
220       1
220       5
223       3
403      -1

My current (simplified) query

SELECT g.gemid as ggemid, g.title as gtitle, gemdetail.filename as gfilename, r.filename as rfilename
FROM (SELECT gems.* FROM gems ) g 
LEFT JOIN 
(SELECT title, x.gemid, x.replygemid, x.userid, y.filename  from gems x 
LEFT JOIN gemdetail y ON x.gemid = y.gemid ) r ON g.gemid = r.replygemid 
LEFT JOIN gemdetail ON g.gemid = gemdetail.gemid 

Results may look like this

ggemid   replygemid gtitle          gfilename                   rfilename
------   ---------- ------          ---------------------       ----------------
220      403        Map             uganda-map.jpg              mona-lisa-x8.jpg
220      403        Map             mozambique-map.jpg          mona-lisa-x8.jpg
223      NULL       Inhabitants     uganda-inhabitants.jpg      NULL
223      NULL       Inhabitants     kenya-inhabitants.jpg       NULL
223      NULL       Inhabitants     kenya-inhabitants.jpg       NULL

Answer 1

I think this is what you want:

SELECT g.gemid as ggemid, g.title as gtitle, gemdetail.filename as gfilename, r.filename as rfilename, rt.sum_rating
FROM (SELECT gems.* FROM gems ) g 
LEFT JOIN 
(SELECT title, x.gemid, x.replygemid, x.userid, y.filename  from gems x 
LEFT JOIN gemdetail y ON x.gemid = y.gemid ) r ON g.gemid = r.replygemid 
LEFT JOIN gemdetail ON g.gemid = gemdetail.gemid 
LEFT JOIN (SELECT gemid, SUM(rating) as sum_rating from rating GROUP BY gemid) rt ON g.gemid = rt.gemid

Answer 2

SQL Fiddle

Query :

SELECT g.gemid as ggemid, g2.gemid as replygemid, 
       g.title as gtitle, gd.filename as gfilename, 
       gd2.filename as rfilename, SUM(rating) as rating
FROM gems g
INNER JOIN gemdetail gd ON g.gemid = gd.gemid
INNER JOIN rating r ON g.gemid = r.gemid
LEFT OUTER JOIN gems g2 ON g.gemid = g2.replygemid
LEFT OUTER JOIN gemdetail gd2 ON g2.gemid = gd2.gemid
GROUP BY g.gemid, g2.gemid, g.title, 
         gd.filename, gd2.filename

Results :

| GGEMID | REPLYGEMID |       GTITLE |              GFILENAME |        RFILENAME | RATING |
|--------|------------|--------------|------------------------|------------------|--------|
|    220 |        403 |          map |     mozambique-map.jpg | mona-lisa-x8.jpg |      6 |
|    220 |        403 |          map |         uganda-map.jpg | mona-lisa-x8.jpg |      6 |
|    223 |     (null) |  inhabitants |  kenya-inhabitants.jpg |           (null) |      3 |
|    223 |     (null) |  inhabitants | uganda-inhabitants.jpg |           (null) |      3 |
|    403 |     (null) | reply to map |       mona-lisa-x8.jpg |           (null) |     -1 |