MySQL查询：Group By 后取出每一组中最大的数据

Posted 2020-12-21 小LUA

环境：MySQL5.7版本

先生成一组测试数据

public static int randAge(){
    return new Random().nextInt(100);
}
public static char randScore(){
    int i = new Random().nextInt(4) + 65;
    return (char)i;
}
public static String randName(){
    StringBuilder s = new StringBuilder();
    for (int i = 0; i < 3; i++){
        s.append((char) (0x4e00 + (int) (Math.random() * (0x9fa5 - 0x4e00 + 1))));
    }
    return s.toString();
}
public static void main(String[] args) {
    for(int i = 1; i <= 20; i++){
        System.out.println("INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES ("+ i +", ‘"+ randName() +"‘, "+ randAge() +", ‘"+  randScore() +"‘);");
    }
}

比如：

INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (1, ‘襠醞奪‘, 70, ‘C‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (2, ‘穟玔檠‘, 77, ‘C‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (3, ‘繜翿宽‘, 11, ‘A‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (4, ‘敕搢謝‘, 55, ‘A‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (5, ‘薰舒翎‘, 86, ‘D‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (6, ‘贿怆軻‘, 31, ‘B‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (7, ‘踰齁陈‘, 26, ‘D‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (8, ‘溧况癈‘, 94, ‘B‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (9, ‘捳縇睢‘, 38, ‘D‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (10, ‘漲堔杙‘, 0, ‘C‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (11, ‘輪塶讣‘, 13, ‘B‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (12, ‘刪庝熽‘, 53, ‘B‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (13, ‘鄀乹聐‘, 90, ‘D‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (14, ‘婈谂畛‘, 52, ‘B‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (15, ‘甔鼖苫‘, 96, ‘D‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (16, ‘刘卟卅‘, 55, ‘A‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (17, ‘够負饶‘, 3, ‘D‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (18, ‘瀠禮翍‘, 14, ‘A‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (19, ‘仉余淳‘, 5, ‘C‘);
INSERT INTO `student`(`id`, `name`, `age`, `score`) VALUES (20, ‘坛唧笶‘, 59, ‘C‘);

思路

1. 先来个最简单的分组，找出每一组最大的

mysql> select score,max(age) age from student group by score;
+-------+-----+
| score | age |
+-------+-----+
| A     |  55 |
| B     |  94 |
| C     |  77 |
| D     |  96 |
+-------+-----+
4 rows in set (0.03 sec)

但是这样无法得知id和name。

2. 来个左连接查询

mysql> select a.id, a.name, a.age, a.score, b.age, b.score from student a left join (select score,max(age) age from student group by score) b on a.score = b.score and a.age = b.age;
+----+--------+-----+-------+------+-------+
| id | name   | age | score | age  | score |
+----+--------+-----+-------+------+-------+
|  4 | 敕搢謝 |  55 | A     |   55 | A     |
| 16 | 刘卟卅 |  55 | A     |   55 | A     |
|  8 | 溧况癈 |  94 | B     |   94 | B     |
|  2 | 穟玔檠 |  77 | C     |   77 | C     |
| 15 | 甔鼖苫 |  96 | D     |   96 | D     |
|  1 | 襠醞奪 |  70 | C     | NULL | NULL  |
|  3 | 繜翿宽 |  11 | A     | NULL | NULL  |
|  5 | 薰舒翎 |  86 | D     | NULL | NULL  |
|  6 | 贿怆軻 |  31 | B     | NULL | NULL  |
|  7 | 踰齁陈 |  26 | D     | NULL | NULL  |
|  9 | 捳縇睢 |  38 | D     | NULL | NULL  |
| 10 | 漲堔杙 |   0 | C     | NULL | NULL  |
| 11 | 輪塶讣 |  13 | B     | NULL | NULL  |
| 12 | 刪庝熽 |  53 | B     | NULL | NULL  |
| 13 | 鄀乹聐 |  90 | D     | NULL | NULL  |
| 14 | 婈谂畛 |  52 | B     | NULL | NULL  |
| 17 | 够負饶 |   3 | D     | NULL | NULL  |
| 18 | 瀠禮翍 |  14 | A     | NULL | NULL  |
| 19 | 仉余淳 |   5 | C     | NULL | NULL  |
| 20 | 坛唧笶 |  59 | C     | NULL | NULL  |
+----+--------+-----+-------+------+-------+
20 rows in set (0.08 sec)

左面的是原数据表的内容，右面多出来的两行是我们分组统计之后的数据。

3. 两个查询调换位置或者用右连接查询，再加个排序

mysql> select a.id, a.name, b.age, b.score from student a right join (select score,max(age) age from student group by score) b on a.score = b.score and a.age = b.age order by b.score;
+----+--------+-----+-------+
| id | name   | age | score |
+----+--------+-----+-------+
|  4 | 敕搢謝 |  55 | A     |
| 16 | 刘卟卅 |  55 | A     |
|  8 | 溧况癈 |  94 | B     |
|  2 | 穟玔檠 |  77 | C     |
| 15 | 甔鼖苫 |  96 | D     |
+----+--------+-----+-------+
5 rows in set (0.07 sec)