CREATE DATABASE mydatabase; USE mydatabase; CREATE TABLE customers ( customerID varchar(8) primary key, name varchar(40) default NULL, phone varchar(16) default NULL ); INSERT INTO customers VALUES ('ADDIFK01','Frank Addinsell', '(718) 555-3911'); INSERT INTO customers VALUES ('ALBIBB01','Bob Albinoni', '(213) 555-7566');在这段代码中package homework7_二_1_20221548; import java.sql.*; import javax.swing.JOptionPane; public class Demo { public static void main(String args[]) { String url = "jdbc:mysql://127.0.0.1:3306/student"; String user = "root", pwd = ""; Connection con; Statement sql; ResultSet rs; try { Class.forName("com.mysql.jdbc.Driver"); } catch (ClassNotFoundException e) { System.out.println("test" + e); } try { con = DriverManager.getConnection(url, user, pwd); sql = con.createStatement(); String sqlStatement; sqlStatement = "SELECT * From message order by height desc"; rs = sql.executeQuery(sqlStatement); while (rs.next()) { double customerID = rs.getDouble("customerID"); String name = rs.getString("name"); double phone = rs.getDouble("phone"); System.out.println(customerID + " " + name + " " + phone); } con.close(); } catch (SQLException e) { System.out.println("数据库操作失败" + e); } } }为什么输不出结果？

这段代码中的 SQL 语句是查询 message 表，但是在数据库中并没有创建名为 message 的表，而是创建了一个名为 customers 的表。因此，需要将 SQL 语句修改为查询 customers 表，同时查询的列名也需要修改为该表中的列名。另外，由于 phone 列的数据类型是 varchar，因此无法直接使用 getDouble() 方法获取，需要改为使用 getString() 方法获取，并且在输出时需要将字符串转换为 double 类型。修改后的代码如下：

package homework7_二_1_20221548;

import java.sql.*;
import javax.swing.JOptionPane;

public class Demo {
    public static void main(String args[]) {
        String url = "jdbc:mysql://127.0.0.1:3306/mydatabase";
        String user = "root", pwd = "";
        Connection con;
        Statement sql;
        ResultSet rs;

        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            System.out.println("test" + e);
        }

        try {
            con = DriverManager.getConnection(url, user, pwd);
            sql = con.createStatement();
            String sqlStatement;
            sqlStatement = "SELECT * From customers order by name";
            rs = sql.executeQuery(sqlStatement);

            while (rs.next()) {
                String customerID = rs.getString("customerID");
                String name = rs.getString("name");
                String phone = rs.getString("phone");
                System.out.println(customerID + " " + name + " " + Double.parseDouble(phone));
            }

            con.close();
        } catch (SQLException e) {
            System.out.println("数据库操作失败" + e);
        }
    }
}

注意，这段代码中的查询语句按照客户姓名升序排列，如果需要按照其他列排序，可以修改 SQL 语句中的 "order by" 子句。