package Test2016;

import java.util.Scanner;

//提示用户输入学生个数，学生姓名和他们的成绩，然后按照学生成绩降序排列;

public class test0225 {

    private String name;

    private long grade;

    public long getGrade(){

        return grade;

    }

    public void setGrade(long grade){

        this.grade=grade;

    }

    public String getName(){

        return name;

    }

    public void setName(String name){

        this.name=name;

    }

    public String toString(){

        return this.name;

    }

     public static void main(String[] args) {

         Scanner input=new Scanner(System.in);

         System.out.println("请输入学生个数");

         int stuNum=input.nextInt();

         test0225 objArr[]=new test0225[stuNum];

         for(int i=0;i<objArr.length;i++){

            objArr[i]=new test0225();

            System.out.println("请输入第"+(i+1)+"个学生的姓名");

            String name=input.next();

            objArr[i].setName(name);

            System.out.println("请输入第"+(i+1)+"个学生的成绩");

            long grade=input.nextLong();

            objArr[i].setGrade(grade);

         }

         System.out.println("打印开始");

         for(int i=0;i<objArr.length;i++){

             for(int j=i+1;j<objArr.length;j++){

                 if(objArr[i].getGrade()<objArr[j].getGrade()){

                test0225 s=new test0225();

                s=objArr[i];

                objArr[i]=objArr[j];

                objArr[j]=s;

                 }

             }

         }

         for(int i=0;i<objArr.length;i++){

             System.out.println(objArr[i].getName()+"的成绩是:"+objArr[i].getGrade());

         }

    }

 }

结果展示：
zhouba	92

zhangsan	83

wangwu	75

lisi	71

zhengshi	69

wujiu	64

yanqi	58

zhaoliu	55
要排序的学生成绩文档：https://pan.baidu.com/s/1cOS_rqAmMg0ioo2dKXFHwQ
代码：
package com.it666.student;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.io.WritableComparator;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

/**
 * @author 吴振峰
 *					编程夜当午，手握小滑鼠。
 *					谁知编程辛，行行皆'心'苦。
 */
 
public class Core2 {
	private static class ScoreA2Mapper extends Mapper<LongWritable, Text, IntWritable, Text>{
		private IntWritable outputkey = new IntWritable();
		private Text outputvalue = new Text();
		@Override
		protected void map(LongWritable key, Text value, Mapper<LongWritable, Text, IntWritable, Text>.Context context)
				throws IOException, InterruptedException {
			
			String line = value.toString();
			String[] words = line.split("\\s+");
			
			String name = words[0];
			int score = Integer.parseInt(words[1]);
			
			// 以成绩为 key，名字为 value 输出
			outputkey.set(score);
			outputvalue.set(name);
			
			context.write(outputkey, outputvalue);
			
		}
		
	}
	
	private static class ScoreA2SortComparator extends WritableComparator{
		@SuppressWarnings("unused")
		public ScoreA2SortComparator() {
			super(IntWritable.class, true);
		}
		
		@SuppressWarnings("rawtypes")
		public int compare(WritableComparable a, WritableComparable b) {
			// 设置排序规则
			// a, b 表示排序算法中需要比较的两个 key，通过返回值控制排序方式
			// 返回值有 3 中情况
			// 返回 0 排序结果是 ab
			// 返回正数 排序结果是 ba
			// 返回负数 排序结果是 ab
			
			// 1. 把 Object 转换为 key 的类型
			IntWritable akey = (IntWritable) a;
			IntWritable bkey = (IntWritable) b;
			
			// 2. 把 key 转换为方便使用的 int 类型
			int aScore = akey.get();
			int bScore = bkey.get();
			
			// 3. 设置排序方式
			if (aScore == bScore) {
				return 0;
			}else if(aScore < bScore) {
				return 1;
			}else {
				return -1;
			}
			
		}
	}
	
	private static class ScoreA2Reducer extends Reducer<IntWritable, Text, Text, IntWritable>{

		@Override
		protected void reduce(IntWritable score, Iterable<Text> names,
				Reducer<IntWritable, Text, Text, IntWritable>.Context context) throws IOException, InterruptedException {
			
			for (Text name : names) {
				context.write(name, score);
			}
			
		}
	}
	
	public static void main(String[] args) {
		
		Configuration conf = new Configuration();
		
		try {
			Job job = Job.getInstance(conf);
			
			job.setJarByClass(Core2.class);
			
			job.setMapperClass(ScoreA2Mapper.class);
			job.setReducerClass(ScoreA2Reducer.class);
			
			// 配置使用自定义分排序方案：ScoreA2SortComparator
			job.setSortComparatorClass(ScoreA2SortComparator.class);
			
			job.setMapOutputKeyClass(IntWritable.class);
			job.setMapOutputValueClass(Text.class);
			
			job.setOutputKeyClass(Text.class);
			job.setOutputValueClass(IntWritable.class);
			
			Path inputPath = new Path("E:/java/MavenHadoopData/04-Students Grade/scoreA.txt");
			FileInputFormat.addInputPath(job, inputPath);
			
			Path outputPath = new Path("E:/java/MavenHadoopData/04-Students Grade/scoreA2");
			FileSystem.get(conf).delete(outputPath, true);
			FileOutputFormat.setOutputPath(job, outputPath);
			
			job.waitForCompletion(true);
			
		} catch (IOException e) {
			e.printStackTrace();
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

学生表student



班级表class



课程表subject



成绩表score



查询所有学生各科成绩按照总成绩降序排列。

1.获取单门课程的学生成绩

select sc.stu_id,sc.score from score sc where sc.subject_id=1001



2.获取所有学生的总成绩

select sc.stu_id,sum(sc.score) sumscore from score sc group by sc.stu_id



3.一共有三门课程，分别获取这三门课程的学生成绩表然后和学生总成绩表联合按总成绩进行降序排列得到一张新表

select a1.stu_id,a1.score 'Java编程',a2.score '应用统计学',a3.score '数据库',b.sumscore '总成绩' from
(select sc.stu_id,sc.score from score sc where sc.subject_id=1001) a1
left join
(select sc.stu_id,sc.score from score sc where sc.subject_id=1002) a2
on a1.stu_id = a2.stu_id left join
(select sc.stu_id,sc.score from score sc where sc.subject_id=1003) a3
on a3.stu_id = a2.stu_id left join
(select sc.stu_id,sum(sc.score) sumscore from score sc group by sc.stu_id) b
on a3.stu_id = b.stu_id 
order by b.sumscore desc

 

4.和学生表联合查询得到学生姓名

select stu.name,c.Java编程,c.应用统计学,c.数据库,c.总成绩 from
(
select a1.stu_id,a1.score 'Java编程',a2.score '应用统计学',a3.score '数据库',b.sumscore '总成绩' from
(select sc.stu_id,sc.score from score sc where sc.subject_id=1001) a1
left join
(select sc.stu_id,sc.score from score sc where sc.subject_id=1002) a2
on a1.stu_id = a2.stu_id left join
(select sc.stu_id,sc.score from score sc where sc.subject_id=1003) a3
on a3.stu_id = a2.stu_id left join
(select sc.stu_id,sum(sc.score) sumscore from score sc group by sc.stu_id) b
on a3.stu_id = b.stu_id order by b.sumscore desc
) c
left join student stu on stu.id = c.stu_id;




得到题目要求的结果。