访问数据库出错

// 示例代码：Java连接MySQL数据库
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class DatabaseConnector {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/mydatabase";
        String user = "username";
        String password = "password";

        try (Connection conn = DriverManager.getConnection(url, user, password)) {
            System.out.println("Connected to the database!");
        } catch (SQLException e) {
            System.err.println("Failed to connect to the database: " + e.getMessage());
        }
    }
}

-- 示例代码：错误的SQL查询
SELECT * FROM users WHERE age = 'twenty'; -- 错误：age是整数类型，不能用字符串'twenty'

-- 正确的SQL查询
SELECT * FROM users WHERE age = 20;

// 示例代码：Java事务处理
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;

public class TransactionExample {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/mydatabase";
        String user = "username";
        String password = "password";

        try (Connection conn = DriverManager.getConnection(url, user, password)) {
            conn.setAutoCommit(false); // 关闭自动提交

            try (PreparedStatement ps1 = conn.prepareStatement("UPDATE users SET balance = balance - 100 WHERE id = 1");
                 PreparedStatement ps2 = conn.prepareStatement("UPDATE users SET balance = balance + 100 WHERE id = 2")) {

                ps1.executeUpdate();
                ps2.executeUpdate();

                conn.commit(); // 提交事务
                System.out.println("Transaction committed successfully!");
            } catch (SQLException e) {
                conn.rollback(); // 回滚事务
                System.err.println("Transaction failed: " + e.getMessage());
            }
        } catch (SQLException e) {
            System.err.println("Failed to connect to the database: " + e.getMessage());
        }
    }
}

-- 示例代码：授予权限
GRANT SELECT, INSERT, UPDATE, DELETE ON mydatabase.users TO 'username'@'localhost';