1.Consider an employee database as shown in figure 4.12. Write a query to find the ID of an employee who does not have a manager. Note that the employee who will not have manager will be an employee ID that is not present in the manages table.

Sure, here's a SQL query to find the ID of an employee who does not have a manager:

SELECT e.ID
FROM employee e
LEFT JOIN manages m ON e.ID = m.employeeID
WHERE m.managerID IS NULL;

Explanation:

- We start by selecting the ID column from the employee table.
- We then use a LEFT JOIN to join the employee table with the manages table on the employeeID column from the employee table and the ID column from the manages table.
- We add a condition to only return rows where the managerID column from the manages table is NULL, indicating that the employee does not have a manager.
- This will return the ID of all employees who do not have a manager, which is what we're looking for.

相关问题

Write a class named ProductSystem, which extends JFrame class. The appearance of the application looks like the following (as shown in Figure 1). 2. At the beginning, input name,price and number at three JTextField fields, then click “Add”button (event source), which binds to a method (event listener) that create a Product object (using name,price and number) and add it into ArrayList<Product>, at the same time display “Add Product xxx sucess!” in JTextArea on the GUI (as shown in Figure 2) 3. When click “Display” button, then read the records from ArrayList , and display the records in JTextArea on the GUI(as shown in Figure 3). 4. When click “Save to File” button, then write the records from ArrayList to file product.txt, at the same time display "file write success" in JTextArea on the GUI (as shown in Figure 4)

以下是一个实现您所需功能的 ProductSystem 类的示例代码：

import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.util.*;
import javax.swing.*;

public class ProductSystem extends JFrame implements ActionListener {
    private JLabel nameLabel, priceLabel, numberLabel;
    private JTextField nameField, priceField, numberField;
    private JButton addButton, displayButton, saveButton;
    private JTextArea outputArea;
    private ArrayList<Product> productList;
    
    public ProductSystem() {
        super("Product System");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(600, 400);
        setLayout(new GridLayout(2, 1));
        
        JPanel inputPanel = new JPanel(new GridLayout(3, 2));
        nameLabel = new JLabel("Name:");
        nameField = new JTextField();
        priceLabel = new JLabel("Price:");
        priceField = new JTextField();
        numberLabel = new JLabel("Number:");
        numberField = new JTextField();
        addButton = new JButton("Add");
        addButton.addActionListener(this);
        inputPanel.add(nameLabel);
        inputPanel.add(nameField);
        inputPanel.add(priceLabel);
        inputPanel.add(priceField);
        inputPanel.add(numberLabel);
        inputPanel.add(numberField);
        inputPanel.add(new JLabel());
        inputPanel.add(addButton);
        
        JPanel outputPanel = new JPanel(new BorderLayout());
        outputArea = new JTextArea();
        outputArea.setEditable(false);
        JScrollPane scrollPane = new JScrollPane(outputArea);
        displayButton = new JButton("Display");
        displayButton.addActionListener(this);
        saveButton = new JButton("Save to File");
        saveButton.addActionListener(this);
        JPanel buttonPanel = new JPanel(new GridLayout(1, 2));
        buttonPanel.add(displayButton);
        buttonPanel.add(saveButton);
        outputPanel.add(scrollPane, BorderLayout.CENTER);
        outputPanel.add(buttonPanel, BorderLayout.SOUTH);
        
        add(inputPanel);
        add(outputPanel);
        
        productList = new ArrayList<Product>();
    }
    
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == addButton) {
            String name = nameField.getText();
            double price = Double.parseDouble(priceField.getText());
            int number = Integer.parseInt(numberField.getText());
            Product product = new Product(name, price, number);
            productList.add(product);
            outputArea.append("Add Product " + product.getName() + " success!\n");
            nameField.setText("");
            priceField.setText("");
            numberField.setText("");
        } else if (e.getSource() == displayButton) {
            outputArea.setText("");
            for (Product product : productList) {
                outputArea.append(product.toString() + "\n");
            }
        } else if (e.getSource() == saveButton) {
            try {
                FileWriter writer = new FileWriter("product.txt");
                for (Product product : productList) {
                    writer.write(product.getName() + "," + product.getPrice() + "," + product.getNumber() + "\n");
                }
                writer.flush();
                writer.close();
                outputArea.append("File write success.\n");
            } catch (IOException ex) {
                System.err.println("Error writing file.");
            }
        }
    }
    
    public static void main(String[] args) {
        ProductSystem productSystem = new ProductSystem();
        productSystem.setVisible(true);
    }
}

class Product {
    private String name;
    private double price;
    private int number;
    
    public Product(String name, double price, int number) {
        this.name = name;
        this.price = price;
        this.number = number;
    }
    
    public String getName() {
        return name;
    }
    
    public double getPrice() {
        return price;
    }
    
    public int getNumber() {
        return number;
    }
    
    public String toString() {
        return name + " $" + price + " x " + number + " = $" + (price * number);
    }
}

该类实现了您所需的所有功能，包括添加产品，显示产品列表和将产品列表保存到文件。在构造函数中，设置了应用程序的标题，大小和布局，并创建了所有必要的组件。在 actionPerformed 方法中，实现了所有按钮的事件处理程序，包括添加按钮，显示按钮和保存到文件按钮。还使用了一个 Product 类来表示产品，并在 toString 方法中生成产品的字符串表示形式。

(Displaying a TicTacToe board) Display a frame that contains nine labels. A labelmay display a cross image icon, a not image icon, or nothing, as shown in Figure 12.18(a). What to display is randomly decided. Use the Math.random()method to generate an integer0,1, or 2, which corresponds to displaying across image icon, a not image icon, or nothing. The cross and not imagescanbe obtained from the cross.gif and not.gif files in the image directory onthe Companion Website.

Here's the code for displaying a TicTacToe board with randomly generated crosses, nots, or nothing in each square:

import javax.swing.*;
import java.awt.*;
import java.util.Random;

public class TicTacToeBoard extends JFrame {
    public TicTacToeBoard() {
        // Set up the frame
        setTitle("Tic Tac Toe Board");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new GridLayout(3, 3));

        // Create an array of labels for the board
        JLabel[] board = new JLabel[9];

        // Create a random number generator
        Random rand = new Random();

        // Loop through the board array and add labels with random icons
        for (int i = 0; i < board.length; i++) {
            ImageIcon icon = null;
            int num = rand.nextInt(3);
            if (num == 0) {
                icon = new ImageIcon("image/cross.gif");
            } else if (num == 1) {
                icon = new ImageIcon("image/not.gif");
            }
            board[i] = new JLabel(icon);
            add(board[i]);
        }

        // Pack and display the frame
        pack();
        setVisible(true);
    }

    public static void main(String[] args) {
        new TicTacToeBoard();
    }
}

Note that this code assumes that the `cross.gif` and `not.gif` files are located in a directory called `image` in the same directory as the compiled Java class. Adjust the file paths accordingly if necessary.