MYSQL score by rank

MySQL

I am using MYSQL to create a rating system to implement my database. What I want to do is to rate each attribute by its percentage. Here is the example database:

ID, value
1, 3
2, 5
3, 2
4, 5

The output I want is:

ID, value, rank, score
1, 3, 2, 5
2, 5, 1, 10
3, 2, 3, 0
4, 5, 1, 10

score's value will based on the rank so it becomes such as

10*(MAX(rank)-(rank))/(MAX(rank)-MIN(rank))

I have done the rank query but stuck with transforming it into scores. Here is the query I got so far:

SELECT `ID`, `value`, FIND_IN_SET( `value`, (
SELECT GROUP_CONCAT(DISTINCT `value` 
ORDER BY `value` DESC)
FROM table) 
) AS rank
FROM table;

Thank you all guys 🙂

Another question:

| ID | VALUE1 | RANK1 | VALUE2 | RANK2 | 
----------------------------------------
|  2 |      5 |     1 |     20 |      2|
|  4 |      5 |     1 |     30 |      1|
|  1 |      3 |     2 |      5 |      4|
|  3 |      2 |     3 |      8 |      3|

How to generate multiple ranking like the table? I have tried

SELECT
    @min_rank := 1 AS min_rank
  , @max_rank1 := (SELECT COUNT(DISTINCT value1) FROM table) AS max_rank1
  , @max_rank2 := (SELECT COUNT(DISTINCT value2) FROM table) AS max_rank2
  ;
SELECT
    ID
  , R1
  , TRUNCATE(5.0+5.0 * (@max_rank1 - R1) / (@max_rank1 - @min_rank), 2) AS Score1
  , R2
  , TRUNCATE(5.0+5.0 * (@max_rank2 - R2) / (@max_rank2 - @min_rank), 2) AS Score2
FROM (
  SELECT
      ID
    , value1
    , FIND_IN_SET( `value1`, (SELECT GROUP_CONCAT(DISTINCT `value1` ORDER BY `value1` DESC) FROM table)) AS R1
    , value2
    , FIND_IN_SET( `value2`, (SELECT GROUP_CONCAT(DISTINCT `value2` ORDER BY `value2` DESC) FROM table)) AS R2
  FROM table
) ranked_table;

It works fine with ranking below 170. My database has approximate 200+ ranking for some values and ranks larger then 170 will be seen as 0 when it returns. In that case, some scores will be miscalculated.

Best Answer

Using the techniques described here to rank items in MySQL, you can calculate the scores you need using a derived table and some hackery with user variables.

Here's your table with the sample data you provided:

CREATE TABLE blah (
    id INT
  , value INT
);

INSERT blah (id, value)
VALUES 
    (1, 3)
  , (2, 5)
  , (3, 2)
  , (4, 5)
;

And here's the query you need:

-- find the min and max ranks for later use
SELECT
    @min_rank := 1 AS min_rank
  , @max_rank := (SELECT COUNT(DISTINCT value) FROM blah) AS max_rank
;

-- use the calculated min and max ranks to calculate score
SELECT
    id
  , value
  , rank
  , 10.0 * (@max_rank - rank) / (@max_rank - @min_rank) AS score
FROM (
  SELECT
      id
    , value
    , @curr_rank := IF(@prev_rank = value, @curr_rank, @curr_rank + 1) AS rank
    , @prev_rank := value
  FROM blah, (SELECT @curr_rank := 0) r, (SELECT @prev_rank := NULL) p
  ORDER BY value DESC
) ranked_blah;

The output I get for the second query on SQL Fiddle is as follows:

| ID | VALUE | RANK | SCORE |
-----------------------------
|  2 |     5 |    1 |    10 |
|  4 |     5 |    1 |    10 |
|  1 |     3 |    2 |     5 |
|  3 |     2 |    3 |     0 |

Note that this approach will work best if you have an index on value. This will let you find @max_rank quickly using the COUNT technique @ypercube described.

Related Solutions

Mysql – Get the rank of a user in a score table

SELECT id, name, score, FIND_IN_SET( score, (
SELECT GROUP_CONCAT( score
ORDER BY score DESC ) 
FROM scores )
) AS rank
FROM scores

gives this list:

id name  score rank
1  Ida   100   2
2  Boo    58   5
3  Lala   88   4
4  Bash  102   1
5  Assem  99   3

Getting a single person score:

SELECT id, name, score, FIND_IN_SET( score, (    
SELECT GROUP_CONCAT( score
ORDER BY score DESC ) 
FROM scores )
) AS rank
FROM scores
WHERE name =  'Assem'

Gives this result:

id name score rank
5 Assem 99 3

You'll have one scan to get the score list, and another scan or seek to do something useful with it. An index on the score column would help performance on large tables.

Mysql – Selecting, ranking and applying a point score based on rank

Sample data:

CREATE TABLE v
    (`vote` varchar(8), `game` int, `year` int)
;

INSERT INTO v
    (`vote`, `game`, `year`)
VALUES
    ('player 1', 2, 2015),
    ('player 1', 2, 2015),
    ('player 2', 2, 2015),
    ('player 2', 2, 2015),
    ('player 2', 2, 2015),
    ('player 3', 2, 2015),
    ('player 1', 2, 2015),
    ('player 3', 2, 2015),
    ('player 4', 2, 2015)
;

Query:

SELECT 
vote,
no_votes,
rank,
3 - rank + (1 / CASE rank WHEN 1 THEN @rc1 WHEN 2 THEN @rc2 WHEN 3 THEN @rc3 END) AS points
FROM (

    SELECT 
    sq.*
    , @rank := if(@prevNoVotes = no_votes, @rank, @rank + 1) AS rank
    , @rc1 := if(@rank = 1, @rc1 + 1, @rc1)
    , @rc2 := if(@rank = 2, @rc2 + 1, @rc2)
    , @rc3 := if(@rank = 3, @rc3 + 1, @rc3)
    , @prevNoVotes := no_votes
    FROM
    (

        SELECT 
        vote, 
        COUNT(*) AS no_votes
        FROM v
        GROUP BY vote

    ) sq
    , (SELECT @rank := 0, @prevNoVotes := null, @rc1 := 0, @rc2 := 0, @rc3 := 0) var_init_subquery
    ORDER BY no_votes DESC

) q

Result:

|     VOTE | NO_VOTES | RANK | POINTS |
|----------|----------|------|--------|
| player 1 |        3 |    1 |    2.5 |
| player 2 |        3 |    1 |    2.5 |
| player 3 |        2 |    2 |      2 |
| player 4 |        1 |    3 |      1 |

see it working live in an sqlfiddle

Explanation:

This

        SELECT 
        vote, 
        COUNT(*) AS no_votes
        FROM v
        GROUP BY vote

is simple enough, I hope :)

This

, (SELECT @rank := 0, @prevNoVotes := null, @rc1 := 0, @rc2 := 0, @rc3 := 0) var_init_subquery

is just a subquery to initialize some variables.

Here

    , @rank := if(@prevNoVotes = no_votes, @rank, @rank + 1) AS rank
    , @rc1 := if(@rank = 1, @rc1 + 1, @rc1)
    , @rc2 := if(@rank = 2, @rc2 + 1, @rc2)
    , @rc3 := if(@rank = 3, @rc3 + 1, @rc3)
    , @prevNoVotes := no_votes

in the first row @prevNoVotes holds the value of the previous row (or the value from the initialization subquery). @rc1, @rc2 and @rc3 are just rank counters. They count for each rank, how many people are on the same rank.
In the last row, the value of the current row is assigned to the variable. When the next row is processed, the variable will hold the value of the previous row in the first line of this part of the code.

And finally in the most outer part of the query we calculate the points distribution.

Feel free to ask any questions :)

UPDATE:

Added this table:

CREATE TABLE points
    (`r` int, `p` int)
;

INSERT INTO points
    (`r`, `p`)
VALUES
    (1, 3),
    (2, 2),
    (3, 1)
;

Added more logic :)

SELECT 
vote,
no_votes,
rank,
@rc := @rc + 1 AS rowcounter,
((SELECT p FROM points WHERE r = IF(rank = @prevRank, rank, @rc)) - 1) + (1 / CASE rank WHEN 1 THEN @rc1 WHEN 2 THEN @rc2 WHEN 3 THEN @rc3 END) AS points,
@prevRank := rank
FROM (
    SELECT 
    sq.*
    , @rank := if(@prevNoVotes = no_votes, @rank, @rank + 1) AS rank
    , @rc1 := if(@rank = 1, @rc1 + 1, @rc1)
    , @rc2 := if(@rank = 2, @rc2 + 1, @rc2)
    , @rc3 := if(@rank = 3, @rc3 + 1, @rc3)
    , @prevNoVotes := no_votes
    FROM
    (

        SELECT 
        vote, 
        COUNT(*) AS no_votes
        FROM v
        GROUP BY vote

    ) sq
    , (SELECT @rank := 0, @prevNoVotes := null, @rc1 := 0, @rc2 := 0, @rc3 := 0) var_init_subquery
    ORDER BY no_votes DESC
)q
, (SELECT @rc := 0, @prevRank := 1) row_counter_init
ORDER BY rank

see it working live in this sqlfiddle

Best Answer

Related Solutions

Mysql – Get the rank of a user in a score table

Mysql – Selecting, ranking and applying a point score based on rank

Related Question