How do I refactor this MySQL query? - mysql

Question

I have following MySQL query:
(SELECT c.Channel as name, count(*) as total_episode
FROM (
SELECT a.aid, a.vid
FROM videoItem v INNER JOIN aid2vid a USING(vid)
GROUP BY a.aid
) a1 INNER JOIN channelListingItem c USING(aid)
GROUP BY c.Channel
)
UNION
(SELECT c1.Channel as name, 0 as total_episode
FROM channelListingItem c1 LEFT JOIN (
SELECT c.Channel FROM (
SELECT a.aid, a.vid
FROM videoItem v INNER JOIN aid2vid a USING(vid)
GROUP BY a.aid
) a1 INNER JOIN channelListingItem c USING(aid)
GROUP BY c.Channel
) c2 USING(Channel)
WHERE c2.Channel is null
GROUP BY name
);
Basically, what this statement does is to get the correct count episode in each channel & assign zero for channels w/o vid in consequent table (videoItem).
Note that
SELECT a.aid, a.vid
FROM videoItem v
INNER JOIN aid2vid a USING(vid)
GROUP BY a.aid
is duplicated twice and from explain this MySQL statement I don't see MySQL re-use the query result.
+----+--------------+------------+------+----------+---------+---------+----------+------+---------------------------------+
| id | select_type | table | type | pos_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+------+----------+---------+---------+----------+------+---------------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 313 | Using temporary; Using filesort |
| 1 | PRIMARY | c | ALL | idx_vid | NULL | NULL | NULL | 616 | Using where; Using join buffer |
| 2 | DERIVED | a | ALL | vid | NULL | NULL | NULL | 1015 | Using temporary; Using filesort |
| 2 | DERIVED | v | ref | idx_vid | idx_vid | 32 | db.a.vid | 10 | Using index |
| 3 | UNION | c1 | ALL | NULL | NULL | NULL | NULL | 616 | Using temporary; Using filesort |
| 3 | UNION | <derived4> | ALL | NULL | NULL | NULL | NULL | 28 | Using where; Not exists |
| 4 | DERIVED | <derived5> | ALL | NULL | NULL | NULL | NULL | 313 | Using temporary; Using filesort |
| 4 | DERIVED | c | ALL | idx_vid | NULL | NULL | NULL | 616 | Using where; Using join buffer |
| 5 | DERIVED | a | ALL | vid | NULL | NULL | NULL | 1015 | Using temporary; Using filesort |
| 5 | DERIVED | v | ref | idx_vid | idx_vid | 32 | db.a.vid | 10 | Using index |
|NULL| UNION RESULT | <union1,3> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------+------------+------+----------+---------+---------+----------+------+---------------------------------+
11 rows in set (0.02 sec)
How do I refactor this MySQL statement? Also is there good refactor tool for MySQL statement?
Thanks.

Answer 1

This one seemed to work for me:
select Channel as name,count(distinct a1.aid) as total_episode
from channelListingItem c
left join
(
select a.aid, a.vid
from videoItem v INNER JOIN aid2vid a USING(vid)
) a1 on a1.aid = c.aid
group by Channel;
From what I can see the following query that you use twice in an inline view:
SELECT a.aid, a.vid
FROM videoItem v INNER JOIN aid2vid a USING(vid)
GROUP BY a.aid
Is being used to get a distinct list of aid and vid values that exist in both videoItem and aid2vid. I have replaced the GROUP BY in the inline view with a COUNT(DISTINCT) to achieve the same thing since you are not using any aggregate functions in the inline view part of the query.
I think you do not need to split the query into two parts joined by a union i.e. part 1 to get episode counts > 0 and part 2 to get episode counts = 0. This can be achieved in one GROUP BY.
Hope this helps!

Answer 2

I could be way of but I believe following to provide the same results as your original query does.
The gist of it is to
Add the total_episode field to your LEFT JOIN.
Use COALESCE to return either the total_episode value or 0.
SQL Statement
SELECT c1.Channel as name
, COALESCE(total_episode, 0)
FROM channelListingItem c1
LEFT JOIN (
SELECT c.Channel
, count(*) as total_episode
FROM (
SELECT a.aid
, a.vid
FROM videoItem v
INNER JOIN aid2vid a ON a.vid = v.vid
GROUP BY
a.aid
) a1
INNER JOIN channelListingItem c ON c.aid = a1.aid
GROUP BY
c.Channel
) c2 ON c2.Channel = c1.Channel
GROUP BY
name