```package com.web.jdbc;

import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.Connection;;

public class demo2 {

public static void main(String[] args) throws Exception {
    // TODO Auto-generated method stub
    DriverManager.registerDriver(new com.mysql.jdbc.Driver());
    Connection conn=DriverManager.getConnection(
            "jdbc:mysql://127.0.0.1:3309/", "root", "123");
    System.out.println(conn!=null?"取得":"未取得");
    //创建封装sql命令的对象
    String sql ="select * from ml";
    Statement stmt=conn.createStatement();
    //执行sql命令，并返回符合条件的记录集合
    ResultSet rs=stmt.executeQuery(sql);
    //处理结果集合
    while(rs.next()) {
        Object obj=rs.getObject("name");
        System.out.println(obj);
    }
    //依次关闭所用过的连接对象
    rs.close();
    stmt.close();
    conn.close();


}

}
运行的话会显示这句话出错 ResultSet rs=stmt.executeQuery(sql);

运行结果：

取得
Exception in thread "main" java.sql.SQLException: No database selected
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:957)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3878)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3814)
    at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2478)
    at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2625)
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2547)
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2505)
    at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1370)
    at com.web.jdbc.demo2.main(demo2.java:21)

实在是不知道咋改了，求大神指点