Question

Pretend I have this table, hypothetically representing users (the “voters”) rating various objects (e.g. places, websites, answers, etc., the “votees”) between one to five stars (the “score”). The more generic version of this question asks what to do for one to N foos instead.

mysql> SHOW CREATE TABLE scores;
CREATE TABLE `scores` (
    `voter_id` BIGINT NOT NULL DEFAULT '0',
    `votee_id` BIGINT NOT NULL DEFAULT '0',
    `score` tinyint NOT NULL,
    PRIMARY KEY (`voter_id`,`votee_id`),
    KEY `votee_index` (`votee_id`)
);

I want to build a .csv file for each votee object, with columns representing the number of one, two, three, four, and five star votes the object has been given, e.g.

output.csv:
votee_id, count_ones, count_twos, count_threes, count_fours, count_fives
1, 3, 7, 5, 3, 2
...

I know I can get the raw data to back this table using this query:

SELECT votee_id, score, COUNT(score)
FROM scores
GROUP BY votee_id, score;

This doesn’t give me the data in the csv format I want thought:

1. it won’t list 0’s for scores not seen for a given object.

2. it doesn’t combine all five scores for an object into a single
   line/row
   (ie, denormalize/histogram the data).

I want to create the output using nothing but mysql.

After hacking around for a while I have the following query, which works; but it’s pretty inefficient, and I can’t find/make anything more elegant:

SELECT votee_id, GROUP_CONCAT(COALESCE(score_count, '0') ORDER BY AllUserCrossScore.score ASC)
FROM
    (SELECT votee_id, score FROM
        (SELECT 1 AS score UNION ALL
         SELECT 2 AS score UNION ALL
         SELECT 3 AS score UNION ALL
         SELECT 4 AS score UNION ALL
         SELECT 5 AS score) ScoresEnum

         JOIN

         (SELECT DISTINCT votee_id FROM scores) DistinctIds
    ) AllUserCrossScore

    LEFT JOIN

    (SELECT votee_id, score, COUNT(score) as score_count
     FROM scores
     GROUP BY votee_id, score
    ) ScoreCounts

    USING (votee_id, score)
GROUP BY votee_id;

In particular, this feels especially hacky because I use GROUP_CONCAT with ‘,’ to put the scores together; and then fiddle elsewhere to have mysql join all the other fields with ‘,’ as well, resulting in the correct formatting for the .csv (fiddling not shown).

How can I do better in a case like this?

Answer 1

I think you are looking for something like this:

SELECT
  votee_id
  SUM(CASE WHEN score = 1 THEN 1 ELSE 0 END) as count_ones
  SUM(CASE WHEN score = 2 THEN 1 ELSE 0 END) as count_twos
  SUM(CASE WHEN score = 3 THEN 1 ELSE 0 END) as count_threes
  SUM(CASE WHEN score = 4 THEN 1 ELSE 0 END) as count_fours
  SUM(CASE WHEN score = 5 THEN 1 ELSE 0 END) as count_fives
FROM scores
GROUP BY votee_id