如何使用java8的stream实现mysql的group by 功能

可以使用Java 8的Stream API和Collectors.groupingBy()方法实现MySQL的group by功能。假设我们有一个包含以下数据的MySQL表：

+----+-------+-------+
| id | name  | score |
+----+-------+-------+
|  1 | Alice |    80 |
|  2 | Bob   |    75 |
|  3 | Alice |    90 |
|  4 | Bob   |    85 |
|  5 | Alice |    95 |
+----+-------+-------+

要按name分组并计算每个组的平均分数，可以使用以下代码：

import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {
    public static void main(String[] args) {
        Stream<Student> students = Stream.of(
                new Student(1, "Alice", 80),
                new Student(2, "Bob", 75),
                new Student(3, "Alice", 90),
                new Student(4, "Bob", 85),
                new Student(5, "Alice", 95)
        );

        Map<String, Double> avgScoresByName = students
                .collect(Collectors.groupingBy(Student::getName, Collectors.averagingDouble(Student::getScore)));

        System.out.println(avgScoresByName);
    }
}

class Student {
    private int id;
    private String name;
    private int score;

    public Student(int id, String name, int score) {
        this.id = id;
        this.name = name;
        this.score = score;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getScore() {
        return score;
    }
}

输出结果为：

{Alice=88.33333333333333, Bob=80.0}

其中，Collectors.groupingBy()方法按照学生的name属性分组，Collectors.averagingDouble()方法计算每个组的平均分数，并返回一个Map<String, Double>对象，其中键为name，值为平均分数。