mysql - 构造二维 MySQL 查询的最有效方法是什么？

Question

我有一个包含以下表和字段的 MySQL 数据库:

• 学生 (id)
• 类(id)
• 年级(id、student_id、class_id、年级)

学生表和类(class)表在 id(主键)上建立索引。成绩表以 id(主键)和 student_id、class_id 和成绩为索引。

我需要构建一个查询，在给定类(class) ID 的情况下，给出所有其他类(class)的列表以及在其他类(class)中得分更高的学生人数。

基本上，给定成绩表中的以下数据:

student_id | class_id | grade
--------------------------------------
1          | 1        | 87
1          | 2        | 91
1          | 3        | 75
2          | 1        | 68
2          | 2        | 95
2          | 3        | 84
3          | 1        | 76
3          | 2        | 88
3          | 3        | 71

用类 ID 1 查询应该产生:

class_id | total
-------------------
2        | 3
3        | 1

理想情况下，我希望它能在几秒钟内执行，因为我希望它成为网络界面的一部分。

我遇到的问题是，在我的数据库中，我有超过 1300 个类(class)和 160,000 名学生。我的成绩表有将近 1500 万行，因此，查询需要很长时间才能执行。

这是我到目前为止尝试过的以及每个查询所用的时间:

-- I manually stopped execution after 2 hours
SELECT    c.id, COUNT(*) AS total
FROM      classes c
              INNER JOIN grades a ON a.class_id = c.id
              INNER JOIN grades b ON b.grade < a.grade AND
                  a.student_id = b.student_id AND
                  b.class_id = 1
WHERE     c.id != 1 AND
GROUP BY  c.id

-- I manually stopped execution after 20 minutes
SELECT    c.id,
          (
              SELECT    COUNT(*) 
              FROM      grades g 
              WHERE     g.class_id = c.id AND g.grade > (
                            SELECT   grade 
                            FROM     grades 
                            WHERE    student_id = g.student_id AND 
                                     class_id = 1
                        )
          ) AS total
FROM      classes c
WHERE     c.id != 1;

-- 1 min 12 sec
CREATE TEMPORARY TABLE temp_blah (student_id INT(11) PRIMARY KEY, grade INT);
INSERT INTO temp_blah SELECT student_id, grade FROM grades WHERE class_id = 1;
SELECT    o.id,
          ( 
              SELECT    COUNT(*)
              FROM      grades g
                            INNER JOIN temp_blah t ON g.student_id = t.student_id
              WHERE     g.class_id = c.id AND t.grade < g.grade
          ) AS total
FROM      classes c
WHERE     c.id != 1;

-- Same thing but with joins instead of a subquery - 1 min 54 sec
SELECT    c.id,
          COUNT(*) AS total
FROM      classes c
              INNER JOIN grades g ON c.id = p.class_id
              INNER JOIN temp_blah t ON g.student_id = t.student_id
WHERE     c.id != 1
GROUP BY  c.id;

我还考虑过创建一个二维表，以学生为行，以类(class)为列，但是我发现了两个问题:

• MySQL 实现了最大列数 (4096) 和最大行大小(以字节为单位)，此方法可能会超过此值
• 我想不出查询该结构以获得我需要的结果的好方法

我还考虑过将这些计算作为后台作业执行并将结果存储在某处，但为了使信息保持最新(必须)，每次创建或更新学生、类(class)或成绩记录时都需要重新计算它们。

有人知道构造此查询的更有效方法吗？

编辑:创建表语句:

CREATE TABLE `classes` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1331 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci$$

CREATE TABLE `students` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=160803 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci$$

CREATE TABLE `grades` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `student_id` int(11) DEFAULT NULL,
  `class_id` int(11) DEFAULT NULL,
  `grade` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `index_grades_on_student_id` (`student_id`),
  KEY `index_grades_on_class_id` (`class_id`),
  KEY `index_grades_on_grade` (`grade`)
) ENGINE=InnoDB AUTO_INCREMENT=15507698 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci$$

最有效查询(1 分 12 秒)的解释输出:

id | select_type        | table | type   | possible_keys                                                             | key                      | key_len | ref               | rows   | extra 
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1  | PRIMARY            | c     | range  | PRIMARY                                                                   | PRIMARY                  | 4       |                   | 683    | Using where; Using index
2  | DEPENDENT SUBQUERY | g     | ref    | index_grades_on_student_id,index_grades_on_class_id,index_grades_on_grade | index_grades_on_class_id | 5       | mydb.c.id         | 830393 | Using where
2  | DEPENDENT SUBQUERY | t     | eq_ref | PRIMARY                                                                   | PRIMARY                  | 4       | mydb.g.student_id | 1      | Using where

另一个编辑 - 解释 sgeddes 建议的输出:

+----+-------------+------------+--------+---------------+------+---------+------+----------+----------------------------------------------+
| id | select_type | table      | type   | possible_keys | key  | key_len | ref  | rows     | Extra                                        |
+----+-------------+------------+--------+---------------+------+---------+------+----------+----------------------------------------------+
|  1 | PRIMARY     | <derived2> | ALL    | NULL          | NULL | NULL    | NULL | 14953992 | Using where; Using temporary; Using filesort |
|  2 | DERIVED     | <derived3> | system | NULL          | NULL | NULL    | NULL |        1 | Using filesort                               |
|  2 | DERIVED     | G          | ALL    | NULL          | NULL | NULL    | NULL | 15115388 |                                              |
|  3 | DERIVED     | NULL       | NULL   | NULL          | NULL | NULL    | NULL |     NULL | No tables used                               |
+----+-------------+------------+--------+---------------+------+---------+------+----------+----------------------------------------------+

Answer 1

我认为这对使用 SUM 和 CASE 的你应该有效:

SELECT C.Id,
  SUM(
    CASE 
    WHEN G.Grade > C2.Grade THEN 1 ELSE 0 
    END
  ) 
FROM Class C
  INNER JOIN Grade G ON C.Id = G.Class_Id
  LEFT JOIN (
      SELECT Grade, Student_Id, Class_Id
      FROM Class
        JOIN Grade ON Class.Id = Grade.Class_Id
      WHERE Class.Id = 1
    ) C2 ON G.Student_Id = C2.Student_Id
WHERE C.Id <> 1
GROUP BY C.Id

Sample Fiddle Demo

--编辑--

针对您的评论，这是另一种应该更快的尝试:

SELECT 
  Class_Id, 
  SUM(CASE WHEN Grade > minGrade THEN 1 ELSE 0 END)
FROM 
(
  SELECT 
    Student_Id,
    @classToCheck:=
      IF(G.Class_Id = 1, Grade, @classToCheck) minGrade ,
    Class_Id,
    Grade
  FROM Grade G
    JOIN (SELECT @classToCheck:= 0) t
  ORDER BY Student_Id, IF(Class_Id = 1, 0, 1)
  ) t
WHERE  Class_Id <> 1
GROUP BY Class_ID

和more sample fiddle .