MySQL+PHP: optimise ranking query and count subquery - php

Question

This is raw data, and want to rank them according to score (count(tbl_1.id)).
[tbl_1]
===========
id | name
===========
1 | peter
2 | jane
1 | peter
2 | jane
3 | harry
3 | harry
3 | harry
3 | harry
4 | ron
So make temporary table (tbl_2) to count score for each id.
SELECT id, name, COUNT( id ) AS score
FROM tbl_1
GROUP BY id
ORDER BY score DESC;
LIMIT 0, 30;
Then result is;
[tbl_2]
===================
id | name | score
===================
3 | harry | 4
1 | peter | 2
2 | jane | 2
4 | ron | 1
Then query this;
SELECT v1.id, v1.name, v1.score, COUNT( v2.score ) AS rank
FROM votes v1
JOIN votes v2 ON v1.score < v2.score
OR (
v1.score = v2.score
AND v1.id = v2.id
)
GROUP BY v1.id, v1.score
ORDER BY v1.rank ASC, v1.id ASC
LIMIT 0, 30;
Then result is;
==========================
id | name | score | rank
==========================
3 | harry | 4 | 1
1 | peter | 2 | 2
2 | jane | 2 | 2
4 | ron | 1 | 4
Is it possible to do this in one transaction (query) nicely?

Answer 1

Yes, it's possible to do this in a single query. But it's a total hairball in MySQL, because MySQL doesn't have a simple ROWNUM operation, and you need one for the rank computation.
Here's your vote query with the rank shown. The #ranka variable is used to number the rows.
SELECT #ranka:=#ranka+1 AS rank, id, name, score
FROM
(
SELECT id,
name,
COUNT( id ) AS score
FROM tbl_1
GROUP BY id
ORDER BY score DESC, id
) votes,
(SELECT #ranka:=0) r
As you have discovered already, you need to self-join this thing to get a proper ranking (which handles ties correctly). So, if you take your query and replace the two references to your votes table each with their own version of this subquery, you get what you need.
SELECT v1.id,
v1.name,
v1.score,
COUNT( v2.score ) AS rank
FROM (
SELECT #ranka:=#ranka+1 AS rank,
id,
name,
score
FROM
(
SELECT id,
name,
COUNT( id ) AS score
FROM tbl_1
GROUP BY id
ORDER BY score DESC, name
) votes,
(SELECT #ranka:=0) r) v1
JOIN (
SELECT #rankb:=#rankb+1 AS rank,
id,
name,
score
FROM
(
SELECT id,
name,
COUNT( id ) AS score
FROM tbl_1
GROUP BY id
ORDER BY score DESC, name
) votes,
(SELECT #rankb:=0) r) v2
ON (v1.score < v2.score) OR
(v1.score = v2.score AND v1.id = v2.id)
GROUP BY v1.id, v1.score
ORDER BY v1.rank ASC, v1.id ASC
LIMIT 0, 30;
Told you it's a hairball. Notice that you need different #ranka and #rankb variables in the two versions of the subquery that you're self-joining, to make the row numbering work correctly: these variables have connection scope, not subquery scope, in MySQL.
http://sqlfiddle.com/#!2/c5350/1/0 shows this working.
Edit: It's far easier to do this using PostgreSQL's RANK() function.
SELECT name, votes, rank() over (ORDER BY votes)
FROM (
SELECT name, count(id) votes
FROM tab
GROUP BY name
)x
http://sqlfiddle.com/#!1/94cca/18/0