Mysql 如何对一组有限的可能值进行反规范化/直方图化输出？

假设我有这张表，假设代表用户（“选民”）对各种对象（如地点、网站、答案等，“选民”）的评级介于一到五星之间（“分数”）。这个问题的更一般的版本是问一对N foos应该做什么

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`)
);

我想为每个votee对象构建一个.csv文件，其中的列表示该对象已获得的一、二、三、四和五星投票数，例如

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

我知道我可以使用以下查询获取原始数据以支持此表：

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

这并没有提供我想要的csv格式的数据，我想：

对于给定对象未看到的分数，它不会列出0

它不会将一个对象的所有五个分数合并为一个分数 行/行 （即，数据的非规范化/直方图化）

我只想使用mysql创建输出

经过一段时间的黑客攻击后，我有以下问题，这是可行的；但它效率很低，我找不到/做得更优雅的东西：

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;

特别是，这让我感觉特别不舒服，因为我使用带有“，”的GROUP_CONCAT将分数组合在一起；然后在别处摆弄，让mysql也用“，”连接所有其他字段，从而为.csv生成正确的格式（摆弄未显示）

---

在这样的情况下，我怎样才能做得更好？

我想你在寻找这样的东西：

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

---

我想你正在寻找这样的东西：

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

---

包装您的原始SQL

SELECT
  votee_id AS votee_id,
  SUM(IF(score=1,count_score,0)) AS count_ones,
  SUM(IF(score=2,count_score,0)) AS count_twos,
  SUM(IF(score=3,count_score,0)) AS count_threes,
  SUM(IF(score=4,count_score,0)) AS count_fours,
  SUM(IF(score=5,count_score,0)) AS count_fives
FROM (
  SELECT votee_id, score, COUNT(score) as count_score
  FROM scores
  GROUP BY votee_id, score
) AS baseview
GROUP BY votee_id;

---

包装您的原始SQL

SELECT
  votee_id AS votee_id,
  SUM(IF(score=1,count_score,0)) AS count_ones,
  SUM(IF(score=2,count_score,0)) AS count_twos,
  SUM(IF(score=3,count_score,0)) AS count_threes,
  SUM(IF(score=4,count_score,0)) AS count_fours,
  SUM(IF(score=5,count_score,0)) AS count_fives
FROM (
  SELECT votee_id, score, COUNT(score) as count_score
  FROM scores
  GROUP BY votee_id, score
) AS baseview
GROUP BY votee_id;

---

这肯定是一个更一般的答案，这使它成为我将记住的答案，尽管对于手头的特定问题，我认为Eric的答案更快，因为它避免了子选择。这肯定是一个更一般的答案，这使它成为我将记住的答案，虽然对于手头的特定问题，我认为Eric的答案更快，因为它避免了子选择。