Question

我的数据库有四个表：student，course，stu_course，score。

课程表的列： id，名称

学生表的列： id，冷杉名称，姓氏，用户名，密码

stu_course 表的列： id，stu_id，course_id （stu_id和cours_id是外键）

列得分表： id，stu_cours_id，得分，日期 （stu_cours_id是ksy的开头）

我的问题是我如何才能在分数表中获得每门课程的四个最后分数？
这是我目前拥有的：

SELECT s.first_name
     , c.name
     , sc.id
     , k.score
     , k.date
     ,  
  FROM student s
  JOIN stu_course sc
    ON sc.stu_id = s.id
  JOIN course c
    ON c.id = sc.course_id
  JOIN score k
    ON k.stu_course_id = sc.id

感谢您的帮助

Answer 1

您可以使用ORDER BY语句对列表进行排序（使用desc的顺序对降序进行排序），然后使用LIMIT x来仅显示前x个结果。

如果您使用ORDER BY DESC LIMIT x，则按您选择的列进行排序时，您会看到最底部的x结果

Answer 2

在MySQL 8.0（Maria DB> = 10.2）中，可以使用窗口函数.setIcon(...)按降序对每门课程的分数进行排名。然后，只过滤掉排名高于4的记录：

import java.awt.*;
import java.awt.event.*;
import java.io.IOException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.*;

@SuppressWarnings("serial")
public class SpriteOnBackground extends JPanel {
    // Image attribution:
    // By Adam Evans - M31, the Andromeda Galaxy (now with h-alpha)
    // Uploaded by NotFromUtrecht, CC BY 2.0, 
    // https://commons.wikimedia.org/w/index.php?curid=12654493
    public static final String ANDROMEDA_IMAGE = "https://upload.wikimedia.org/wikipedia/commons/"
            + "thumb/9/98/Andromeda_Galaxy_%28with_h-alpha%29.jpg/"
            + "1280px-Andromeda_Galaxy_%28with_h-alpha%29.jpg";
    public static final String SPRITE_IMAGE = "https://upload.wikimedia.org/wikipedia/commons/"
            + "thumb/a/a1/Glossy_3d_blue_blue2.png/"
            + "120px-Glossy_3d_blue_blue2.png";
    private Image background;
    private JLabel spriteLabel = new JLabel();

    public SpriteOnBackground(Image bg, Image spriteImg) {
        background = bg;
        spriteLabel.setIcon(new ImageIcon(spriteImg));
        spriteLabel.setSize(spriteLabel.getPreferredSize());
        setLayout(null);
        add(spriteLabel);

        MyMouse myMouse = new MyMouse();
        spriteLabel.addMouseListener(myMouse);
        spriteLabel.addMouseMotionListener(myMouse);
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet() || background == null) {
            return super.getPreferredSize();
        }
        // make JPanel the size of the image
        int w = background.getWidth(this);
        int h = background.getHeight(this);
        return new Dimension(w, h);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        // draw background image
        g.drawImage(background, 0, 0, this);
    }

    // mouse listener to drag the JLabel around the GUI
    private class MyMouse extends MouseAdapter {
        private Point p1;
        private Point pSprite;

        @Override
        public void mousePressed(MouseEvent e) {
            p1 = e.getLocationOnScreen();
            pSprite = spriteLabel.getLocation();
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (p1 != null) {
                moveSprite(e);
            }
        }

        private void moveSprite(MouseEvent e) {
            Point p2 = e.getLocationOnScreen();
            int x = pSprite.x + p2.x - p1.x;
            int y = pSprite.y + p2.y - p1.y;
            Point newP = new Point(x, y);
            spriteLabel.setLocation(newP);
            repaint();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            if (p1 != null) {
                moveSprite(e);
            }
            p1 = null;
        }
    }

    private static void createAndShowGui() {
        SpriteOnBackground mainPanel = null;
        try {
            URL backgroundUrl = new URL(ANDROMEDA_IMAGE);
            Image backGroundImg = ImageIO.read(backgroundUrl);

            URL spriteUrl = new URL(SPRITE_IMAGE);
            Image spriteImg = ImageIO.read(spriteUrl);
            mainPanel = new SpriteOnBackground(backGroundImg, spriteImg);
        } catch (IOException e) {
            e.printStackTrace();
        }

        JFrame frame = new JFrame("Sprite On Background");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

在早期版本的MySQL / MariaDB中，一种解决方案是使用相关子查询：

ROW_NUMBER()

我如何在mysql中获得每门课程的四个最后分数？

2 个答案: