String query = "SELECT * FROM score WHERE id = ?"; ps = connection.prepareStatement(query);是什么意思

这段代码是在创建一个预编译的SQL查询语句其中包含一个参数符。

首先，了一个查询语句字符串 `SELECT * FROM WHERE id = ?`，其中 `?` 是一个占位符，表示该位置将由实际值替代。

然后通过 `connection.prepareStatement(query 方法创建一个 `PreparedStatement` 对象 ``，该对象用于执行预编译的查询语句。

在预编译的查询语句中，`id = ?` 部分表示我们希望通过 `id` 列进行查询，并且这个条件的值将由后续的代码动态地设置。

通过使用预编译的语句和参数占位符，可以提高查询性能，并且避免了潜在的SQL注入攻击。在执行查询之前，我们可以使用 `ps.setInt(index, value)` 等方法为占位符设置实际的值。

例如，如果要设置 `id` 的值为 123，可以使用以下代码：

ps.setInt(1, 123);

然后，可以通过执行 `ps.executeQuery()` 方法来执行查询操作，并获取结果集。

相关问题

java假定您已经有一个包含学生信息的 students 表，其中包括每个学生的姓名、年龄和成绩，请编写一个程序： 连接到名为 mydatabase 的 MySQL 数据库。 查询所有学生的信息并将其打印出来。 向表中插入一条新记录，包括姓名为 "张三"，年龄为 20 岁，成绩为 80 分。 更新姓名为 "张三" 的学生的成绩为 90 分。 删除姓名为 "李四" 的学生记录。

好的，根据您的要求，我给您提供以下 Java 代码：

import java.sql.*;

public class StudentDatabase {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/mydatabase?useSSL=false";
        String user = "root";
        String password = "password";
        
        try {
            Connection connection = DriverManager.getConnection(url, user, password);
            
            // 查询所有学生信息
            String query = "SELECT * FROM students";
            Statement statement = connection.createStatement();
            ResultSet resultSet = statement.executeQuery(query);
            
            while(resultSet.next()) {
                String name = resultSet.getString("name");
                int age = resultSet.getInt("age");
                int score = resultSet.getInt("score");
                System.out.println("Name: " + name + ", Age: " + age + ", Score: " + score);
            }
            
            // 插入新记录
            String insert = "INSERT INTO students (name, age, score) VALUES (?, ?, ?)";
            PreparedStatement preparedStatement = connection.prepareStatement(insert);
            preparedStatement.setString(1, "张三");
            preparedStatement.setInt(2, 20);
            preparedStatement.setInt(3, 80);
            preparedStatement.executeUpdate();
            
            // 更新记录
            String update = "UPDATE students SET score = ? WHERE name = ?";
            preparedStatement = connection.prepareStatement(update);
            preparedStatement.setInt(1, 90);
            preparedStatement.setString(2, "张三");
            preparedStatement.executeUpdate();
            
            // 删除记录
            String delete = "DELETE FROM students WHERE name = ?";
            preparedStatement = connection.prepareStatement(delete);
            preparedStatement.setString(1, "李四");
            preparedStatement.executeUpdate();
            
            connection.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

需要注意的是，在运行此程序之前，您需要将 MySQL 驱动程序添加到您的类路径中。

用java代码+MySQL代码实现上述问题

好的，下面是一个简单的Java代码实现学生选课系统的示例，使用了Java JDBC连接MySQL数据库：

1. 首先需要创建数据库和数据表，可以使用以下SQL语句：

CREATE DATABASE `course_selection` CHARACTER SET utf8 COLLATE utf8_general_ci;

USE `course_selection`;

CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  `gender` varchar(10) NOT NULL,
  `age` int(11) NOT NULL,
  `major` varchar(50) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

CREATE TABLE `course` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  `teacher` varchar(50) NOT NULL,
  `description` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

CREATE TABLE `teacher` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  `gender` varchar(10) NOT NULL,
  `age` int(11) NOT NULL,
  `course` varchar(50) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

CREATE TABLE `course_selection` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `student_id` int(11) NOT NULL,
  `course_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `student_id` (`student_id`),
  KEY `course_id` (`course_id`),
  CONSTRAINT `course_selection_ibfk_1` FOREIGN KEY (`student_id`) REFERENCES `student` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `course_selection_ibfk_2` FOREIGN KEY (`course_id`) REFERENCES `course` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

2. 然后创建Java项目，引入MySQL连接驱动jar包，可以使用以下代码连接MySQL数据库：

public class DatabaseUtil {
    private static final String URL = "jdbc:mysql://localhost:3306/course_selection?useUnicode=true&characterEncoding=utf8&serverTimezone=GMT%2B8&useSSL=false";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "123456";
    private static Connection conn = null;

    static {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
    }

    public static Connection getConnection() {
        return conn;
    }
}

3. 接下来就可以实现各个功能模块的Java代码了，例如：

学生信息管理：

public class StudentDao {
    public void add(Student student) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "INSERT INTO student(name, gender, age, major) VALUES (?, ?, ?, ?)";
            ps = conn.prepareStatement(sql);
            ps.setString(1, student.getName());
            ps.setString(2, student.getGender());
            ps.setInt(3, student.getAge());
            ps.setString(4, student.getMajor());
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public void delete(int id) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "DELETE FROM student WHERE id=?";
            ps = conn.prepareStatement(sql);
            ps.setInt(1, id);
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public void update(Student student) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "UPDATE student SET name=?, gender=?, age=?, major=? WHERE id=?";
            ps = conn.prepareStatement(sql);
            ps.setString(1, student.getName());
            ps.setString(2, student.getGender());
            ps.setInt(3, student.getAge());
            ps.setString(4, student.getMajor());
            ps.setInt(5, student.getId());
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public List<Student> queryAll() {
        List<Student> list = new ArrayList<>();
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            String sql = "SELECT * FROM student";
            ps = conn.prepareStatement(sql);
            rs = ps.executeQuery();
            while (rs.next()) {
                Student student = new Student();
                student.setId(rs.getInt("id"));
                student.setName(rs.getString("name"));
                student.setGender(rs.getString("gender"));
                student.setAge(rs.getInt("age"));
                student.setMajor(rs.getString("major"));
                list.add(student);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (rs != null) {
                    rs.close();
                }
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return list;
    }
}

选课管理：

public class CourseSelectionDao {
    public void add(CourseSelection courseSelection) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "INSERT INTO course_selection(student_id, course_id) VALUES (?, ?)";
            ps = conn.prepareStatement(sql);
            ps.setInt(1, courseSelection.getStudentId());
            ps.setInt(2, courseSelection.getCourseId());
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public void delete(int id) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "DELETE FROM course_selection WHERE id=?";
            ps = conn.prepareStatement(sql);
            ps.setInt(1, id);
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public List<CourseSelection> queryAll() {
        List<CourseSelection> list = new ArrayList<>();
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            String sql = "SELECT * FROM course_selection";
            ps = conn.prepareStatement(sql);
            rs = ps.executeQuery();
            while (rs.next()) {
                CourseSelection courseSelection = new CourseSelection();
                courseSelection.setId(rs.getInt("id"));
                courseSelection.setStudentId(rs.getInt("student_id"));
                courseSelection.setCourseId(rs.getInt("course_id"));
                list.add(courseSelection);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (rs != null) {
                    rs.close();
                }
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return list;
    }
}

成绩管理：

public class ScoreDao {
    public void add(Score score) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "INSERT INTO score(student_id, course_id, score) VALUES (?, ?, ?)";
            ps = conn.prepareStatement(sql);
            ps.setInt(1, score.getStudentId());
            ps.setInt(2, score.getCourseId());
            ps.setInt(3, score.getScore());
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public void update(Score score) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "UPDATE score SET score=? WHERE student_id=? AND course_id=?";
            ps = conn.prepareStatement(sql);
            ps.setInt(1, score.getScore());
            ps.setInt(2, score.getStudentId());
            ps.setInt(3, score.getCourseId());
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public List<Score> queryAll() {
        List<Score> list = new ArrayList<>();
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            String sql = "SELECT * FROM score";
            ps = conn.prepareStatement(sql);
            rs = ps.executeQuery();
            while (rs.next()) {
                Score score = new Score();
                score.setId(rs.getInt("id"));
                score.setStudentId(rs.getInt("student_id"));
                score.setCourseId(rs.getInt("course_id"));
                score.setScore(rs.getInt("score"));
                list.add(score);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (rs != null) {
                    rs.close();
                }
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close