Java实现mysql横向数据面试题

在平时开发、学习、面试中，经常会遇到一些数据是需要根据数据生成字段的。就是我们常说的横向显示数据。

最近楼主运到了一个面试题，发现面试和实际工作的做法有点不同。

CREATE TABLE `tablea` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `course` varchar(20) DEFAULT NULL,
  `grade` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;复制代码

楼主将用两种解题思路来做这道题目(ps:一个是面试，一个是工作)

题目如下:

​

解题思路

1.面试中

根据 name 排序, 在根据需求把横向数据定义死。(缺点：课程名称定义死了，不利于扩展)

select name ,
max(case course when  '语文' then grade else 0 end) 语文,
max(case course when  '数学' then grade else 0 end) 数学,
max(case course when  '物理' then grade else 0 end) 物理 
from tablea 
group by name

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2.实际工作中

我的思路是

第一步获取所有数据

​

第二步去重分别得到 横向的课程数据集合，和纵向的第一列学生名称的集合

​

第三步 遍历 学生名称集合

​

第四步 在学生名称集合里面遍历课程数据集合

然后根据 学生名称 和 课程名称 去所有 数据集合 找到 成绩

最后关联到学生的上去

​

第五步 利用map 封装数据 放回给前端

​

最后面生成json数据是这样的

{
    "code": 200,
    "data": {
        "subject": [
            "语文",
            "数学",
            "物理"
        ],
        "students": [
            {
                "grades": [
                    {
                        "course": "语文",
                        "grade": 81
                    },
                    {
                        "course": "数学",
                        "grade": 83
                    },
                    {
                        "course": "物理",
                        "grade": 100
                    }
                ],
                "name": "张三"
            },
            {
                "grades": [
                    {
                        "course": "语文",
                        "grade": 74
                    },
                    {
                        "course": "数学",
                        "grade": 84
                    },
                    {
                        "course": "物理",
                        "grade": 100
                    }
                ],
                "name": "李四"
            }
        ]
    },
    "msg": "success"
}复制代码

最后把接口给前端调用渲染

​

我在数据中添加了一个英语课程和数据，就自动扩展了

​

话不多说，直接上代码

bean包下面

package com.itbbs.bean;

/**
 * @author tjx
 *
 * @param <T>
 * 
 * 公共返回类
 */
public class ComResponseBean<T> {
	
	private String msg ;
	
	private int code;	

	private T data;

	public String getMsg() {
		return msg;
	}

	public void setMsg(String msg) {
		this.msg = msg;
	}

	public int getCode() {
		return code;
	}

	public void setCode(int code) {
		this.code = code;
	}

	public T getData() {
		return data;
	}

	public void setData(T data) {
		this.data = data;
	}
}
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pojo包下面

package com.itbbs.pojo;

/**
 * @作者: tjx
 * @描述: 成绩 (科目 对应 成绩)
 * @创建时间: 创建于11:56 2018/9/26
 **/
public class Grade {

    public Grade(String course, Integer grade) {
        this.course = course;
        this.grade = grade;
    }


    /**
     * 课程
     */
    private String course;

    /**
     * 成绩
     */
    private Integer grade;


    public String getCourse() {
        return course;
    }

    public void setCourse(String course) {
        this.course = course;
    }

    public Integer getGrade() {
        return grade;
    }

    public void setGrade(Integer grade) {
        this.grade = grade;
    }
}



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package com.itbbs.pojo;

import java.util.List;

/**
 * @作者: tjx
 * @描述: 学生
 * @创建时间: 创建于14:10 2018/9/26
 **/
public class Student {
    
    
    /**
     * 学生名称
     */
    private String name;
    
    private List<Grade> grades;

    public List<Grade> getGrades() {
        return grades;
    }

    public void setGrades(List<Grade> grades) {
        this.grades = grades;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}
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package com.itbbs.pojo;

import java.util.List;

/**
 * @作者: tjx
 * @描述: 学生成绩
 * @创建时间: 创建于11:14 2018/9/26
 **/
public class StudentGrade {

    private int id;

    /**
     * 学生名称
     */
    private String name;

    /**
     * 课程
     */
    private String course;

    /**
     * 成绩
     */
    private Integer grade;


    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCourse() {
        return course;
    }

    public void setCourse(String course) {
        this.course = course;
    }

    public Integer getGrade() {
        return grade;
    }

    public void setGrade(Integer grade) {
        this.grade = grade;
    }

}
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dao包下面

package com.itbbs.dao;

import com.itbbs.pojo.StudentGrade;

import java.util.ArrayList;
import java.util.List;

/**
 * @作者: tjx
 * @描述:  模拟dao
 * @创建时间: 创建于11:16 2018/9/26
 **/
public class StudentGradeDAO {

    public List<StudentGrade> selectAll(){
        List<StudentGrade> list = new ArrayList<>();

        StudentGrade studentGrade1 = new StudentGrade();
        studentGrade1.setId(1);
        studentGrade1.setName("张三");
        studentGrade1.setCourse("语文");
        studentGrade1.setGrade(81);

        StudentGrade studentGrade2 = new StudentGrade();
        studentGrade2.setId(2);
        studentGrade2.setName("张三");
        studentGrade2.setCourse("数学");
        studentGrade2.setGrade(83);

        StudentGrade studentGrade3 = new StudentGrade();
        studentGrade3.setId(3);
        studentGrade3.setName("张三");
        studentGrade3.setCourse("物理");
        studentGrade3.setGrade(93);

        StudentGrade studentGrade4 = new StudentGrade();
        studentGrade4.setId(4);
        studentGrade4.setName("李四");
        studentGrade4.setCourse("语文");
        studentGrade4.setGrade(74);


        StudentGrade studentGrade5 = new StudentGrade();
        studentGrade5.setId(5);
        studentGrade5.setName("李四");
        studentGrade5.setCourse("数学");
        studentGrade5.setGrade(84);


        StudentGrade studentGrade6 = new StudentGrade();
        studentGrade6.setId(6);
        studentGrade6.setName("李四");
        studentGrade6.setCourse("物理");
        studentGrade6.setGrade(94);

        //添加外语科目
        StudentGrade studentGrade7 = new StudentGrade();
        studentGrade7.setId(6);
        studentGrade7.setName("张三");
        studentGrade7.setCourse("物理");
        studentGrade7.setGrade(100);

        StudentGrade studentGrade8 = new StudentGrade();
        studentGrade8.setId(6);
        studentGrade8.setName("李四");
        studentGrade8.setCourse("物理");
        studentGrade8.setGrade(100);

        list.add(studentGrade1);
        list.add(studentGrade2);
        list.add(studentGrade3);
        list.add(studentGrade4);
        list.add(studentGrade5);
        list.add(studentGrade6);
        list.add(studentGrade7);
        list.add(studentGrade8);

        return list;
    }

}
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service包下面

package com.itbbs.service;

import com.itbbs.bean.ComResponseBean;
import com.itbbs.dao.StudentGradeDAO;
import com.itbbs.pojo.Grade;
import com.itbbs.pojo.Student;
import com.itbbs.pojo.StudentGrade;
import com.itbbs.utils.ArrayListUtil;

import java.util.*;
import java.util.stream.Collectors;

/**
 * @作者: tjx
 * @描述:  成绩模块业务层
 * @创建时间: 创建于14:27 2018/9/26
 **/
public class StudentGradeService {

    //此处模拟dao
    StudentGradeDAO dao = new StudentGradeDAO();


    public ComResponseBean gradeList(){
        //查询所有数据
        List<StudentGrade> data = dao.selectAll();
        //使用steam 去重 获取所有的科目
        List<StudentGrade> courses = data.stream()
                .filter(ArrayListUtil.distinctByKey(p -> p.getCourse())) //去重
                .collect(Collectors.toList());
        //使用steam 分组 获取所有学生
        List<StudentGrade> names = data.stream()
                .filter(ArrayListUtil.distinctByKey(p -> p.getName()))  //去重
                .collect(Collectors.toList());



        //结果集
        List<Student> students = new ArrayList<>();

        List<String>  subject  = new ArrayList<>();
        courses.forEach(course->subject.add(course.getCourse()));

        //根据学生成绩找到 对应的科目成绩
        names.forEach(student->{
            //获取学生名称
            String name = student.getName();
            List<Grade> grades = new ArrayList<>();
            //遍历科目找到 改学生所有科目成绩
            courses.forEach(course->{
                //获取科目
                String courseName = course.getCourse();
                //根据 学生名称 和 学生科目 筛选出符合条件的数据
                StudentGrade studentGrade = data.stream()
                        .filter(p -> p.getName().equals(name) && p.getCourse().equals(courseName))  //筛选条件
                        .sorted(Comparator.comparing(StudentGrade::getGrade).reversed()) //根据筛选出来的结果进行排序
                        .findFirst().orElse(null);//获取 排序 后的第一个（也就是最大的）
                //找到符合条件的成绩
                grades.add(new Grade(courseName,studentGrade.getGrade()));
            });
            //创建学生类
            Student stu = new Student();
            stu.setGrades(grades);
            stu.setName(name);
            students.add(stu);
        });
        Map result = new HashMap<>();
        result.put("subject",subject);
        result.put("students",students);
        ComResponseBean bean = new ComResponseBean();
        bean.setCode(200);
        bean.setMsg("success");
        bean.setData(result);
        return bean;
    }
}
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utils包下

package com.itbbs.utils;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.function.Predicate;

public class ArrayListUtil {


	/**
	 * 去重复元素
	 * @param keyExtractor
	 * @param <T>
	 * @return
	 */
	public static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
		Map<Object, Boolean> map = new ConcurrentHashMap<>();
		return t -> map.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
	}
}
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html代码

<!DOCTYPE html>
<html>

	<head>
		<meta charset="utf-8">
		<title>学生成绩</title>
		<link rel="stylesheet" href="https://cdn.bootcss.com/bootstrap/3.3.7/css/bootstrap.min.css">
		<script src="https://cdn.bootcss.com/jquery/2.1.1/jquery.min.js"></script>
		<script src="https://cdn.bootcss.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
	</head>

	<body>

		<table class="table table-striped">
			<h1 style="text-align: center;">学生成绩</h1>
			<thead id="thead">
				
			</thead>
			<tbody id="tbody">
				
			</tbody>
		</table>
		<script type="text/javascript">
			//此处模拟ajax请求数据
			function ajax() {
				return {"code":200,"data":{"subject":["语文","数学","物理","外语"],"students":[{"grades":[{"course":"语文","grade":81},{"course":"数学","grade":83},{"course":"物理","grade":93},{"course":"外语","grade":60}],"name":"张三"},{"grades":[{"course":"语文","grade":74},{"course":"数学","grade":84},{"course":"物理","grade":94},{"course":"外语","grade":70}],"name":"李四"}]},"msg":"success"}
			}
			//调用ajax获取数据
			var data = ajax();
			if(data.code == 200){
				//渲染表头
				var subject = data.data.subject;
				var subjectSize  = subject.length;
				var thead = $("#thead");
				var th = "<tr><td>学生姓名</td>"
				for (var i=0;i<subjectSize;i++) {
					th+="<td>" + subject[i]+"</td>";
				}
				th += "</tr>";
				thead.html(th);
				//渲染表身体	
				var students = data.data.students;
				var studentSize = students.length;
				var tbody = $("#tbody");
				var tb = '';
				for (var i=0;i<studentSize;i++) {
					//获取名称
					tb+="<tr><td>"+students[i].name+"</td>";
					var grades = students[i].grades;
					//获取报名项目
					for (var j=0;j<subjectSize;j++) {
						//获取课程
						var course = subject[j];
						//获取改名称下的参赛名
						for (var k=0;k<grades.length;k++) {
							if(course == grades[k].course){
								tb += "<td>"+grades[k].grade+"</td>";
								continue;
							}
						}
					}
				}
				tbody.html(tb);
			}else{
				alert("加载失败")
			}
		</script>

	</body>

</html>复制代码