PHP：如何在单个查询中优化这些MySQL查询？

Question

Is it possible to optimize the following into a single query?

I am assuming here that a single query would be more efficient that multiple queries using a temporary table, so please let me know if my assumption is incorrect.

$id is the current memberid. $list is a list of itemids to be removed from the final results (eg items already downloaded).

What this query is supposed to do it find the top 500 members who have downloaded similar items to the $id member. Then find all the items that these members have downloaded, ranked by a score based on the number of similar downloads from each member and the total number of downloads of each item. The final result is therefore a list of recommendations for the $id member.

The queries are:

mysql_query('CREATE TEMPORARY TABLE temp1 ENGINE=MEMORY AS
(SELECT a.memberid, COUNT(*) `score` FROM table_downloads a INNER JOIN
(SELECT itemid FROM table_downloads WHERE memberid='.$id.') b ON a.itemid = b.itemid
WHERE a.memberid!='.$id.' GROUP BY a.memberid HAVING score>0 ORDER BY score DESC LIMIT 500)');

$res=mysql_query('SELECT table_downloads.itemid,COUNT(table_downloads.itemid*temp1.score) AS score2
FROM table_downloads,temp1
WHERE table_downloads.memberid=temp1.memberid AND table_downloads.itemid NOT IN ('.$list.') 
GROUP BY table_downloads.itemid
ORDER BY score2 DESC LIMIT 30');

mysql_query('DROP TABLE temp1');

It's possible that this query might take too long as to be unusable if there were several million rows. Any advice on ensuring it is executes quickly would also be greatly appreciated.

* I am using mysql_query deliberately. Please do not tell me to use mysqli.*

Answer 1

1. It's not possible to do with mysql_query()
2. It's a mistake to think that joining 3 calls into one would save something noticeable in this case

And to be clear it's a very common delusion - to think that a single messy query would run faster than multiple. It wouldn't.

Answer 2

Out of interest I had a play.

I think it is possible, although not pretty and I suspect slower than your current script.

SELECT table_downloads.itemid, COUNT(table_downloads.itemid * temp1.score) AS score2
FROM table_downloads
INNER JOIN (SELECT a.memberid, COUNT(*) `score` 
            FROM table_downloads a 
            INNER JOIN table_downloads b
            ON  a.itemid = b.itemid AND b.memberid='.$id.' AND a.memberid=b.memberid
            GROUP BY a.memberid 
            ORDER BY score DESC
            LIMIT 500) temp1
ON table_downloads.memberid = temp1.memberid 
WHERE table_downloads.itemid NOT IN ('.$list.') 
GROUP BY table_downloads.itemid
ORDER BY score2 DESC 
LIMIT 30

Not really tested though (don't know your table layouts)