Question

现在我在java中学习ResultSet类型。在这里，我编写了以不同方式查看记录的代码。首先，我显示了emp4表中的所有记录，然后我开始以不同的方式查看这些记录（上一个，第一个，下一个）这正是我正在寻找但它不会显示所有呈现的记录在emp4表中。看到第一个程序，它不起作用，但如果我记录第41行（见第二个程序中的这个），它只是工作真棒。有什么问题？我的代码有什么问题???

代码示例1

package demojdbc;

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

public class MysqlCon{

private static final String DB_DRIVER = "com.mysql.jdbc.Driver";
private static final String DB_CONNECTION = "jdbc:mysql://localhost:3306/vinoth";
private static final String DB_USER = "root";
private static final String DB_PASSWORD = "vino";

    public static void main(String args[])throws SQLException{

        //Creating statement and connection 
        Connection dbConnection = null;
        Statement stmt = null;

        try{

            //Creating class driver
            Class.forName(DB_DRIVER);

            //Creating Database Connection
            dbConnection = DriverManager.getConnection(DB_CONNECTION,DB_USER,DB_PASSWORD);

            //Creating statement
            stmt = dbConnection.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);

            //Creating query
            String sql = "SELECT id,gmail,yahoo from emp4";

            //Creating ResultSet
            ResultSet rs = stmt.executeQuery(sql);

            //Displaying database
            System.out.println("Displaying records before doing some operations");
            System.out.println(rs.getInt(1)+" "+rs.getString(2)+" "+rs.getString(3));

            System.out.println("Displaying records for last row");
            rs.last();

            int id = rs.getInt("id");
            String gmail = rs.getString("gmail");
            String yahoo = rs.getString("yahoo");

            //Displaying records in last row
            System.out.println("ID : "+id);
            System.out.println("GMAIL : "+gmail);
            System.out.println("YAHOO : "+yahoo);

            System.out.println();
            rs.first();
            System.out.println("Displaying records for first row");

            id = rs.getInt("id");
            gmail = rs.getString("gmail");
            yahoo = rs.getString("yahoo");

            //Displaying records in last row
            System.out.println("ID : "+id);
            System.out.println("GMAIL : "+gmail);
            System.out.println("YAHOO : "+yahoo);

            System.out.println();
            rs.next();
            System.out.println("Displaying records for next row");

            id = rs.getInt("id");
            gmail = rs.getString("gmail");
            yahoo = rs.getString("yahoo");

            //Displaying records in last row
            System.out.println("ID : "+id);
            System.out.println("GMAIL : "+gmail);
            System.out.println("YAHOO : "+yahoo);

        }catch(SQLException e){

            e.printStackTrace();

        }catch(ClassNotFoundException e){

             System.out.println("Plese check the driver class path "+e.getMessage());

        }finally{

            if(stmt != null){

                stmt.close();

            }
            if(dbConnection != null){

                dbConnection.close();

            }
        }
    }
}

在这里，代码将正常工作.....

代码示例2

package demojdbc;

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

public class MysqlCon{

private static final String DB_DRIVER = "com.mysql.jdbc.Driver";
private static final String DB_CONNECTION = "jdbc:mysql://localhost:3306/vinoth";
private static final String DB_USER = "root";
private static final String DB_PASSWORD = "vino";

    public static void main(String args[])throws SQLException{

        //Creating statement and connection 
        Connection dbConnection = null;
        Statement stmt = null;

        try{

            //Creating class driver
            Class.forName(DB_DRIVER);

            //Creating Database Connection
            dbConnection = DriverManager.getConnection(DB_CONNECTION,DB_USER,DB_PASSWORD);

            //Creating statement
            stmt = dbConnection.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);

            //Creating query
            String sql = "SELECT id,gmail,yahoo from emp4";

            //Creating ResultSet
            ResultSet rs = stmt.executeQuery(sql);

            //Displaying database
            System.out.println("Displaying records before doing some operations");
           //System.out.println(rs.getInt(1)+" "+rs.getString(2)+" "+rs.getString(3));

            System.out.println("Displaying records for last row");
            rs.last();

            int id = rs.getInt("id");
            String gmail = rs.getString("gmail");
            String yahoo = rs.getString("yahoo");

            //Displaying records in last row
            System.out.println("ID : "+id);
            System.out.println("GMAIL : "+gmail);
            System.out.println("YAHOO : "+yahoo);

            System.out.println();
            rs.first();
            System.out.println("Displaying records for first row");

            id = rs.getInt("id");
            gmail = rs.getString("gmail");
            yahoo = rs.getString("yahoo");

            //Displaying records in last row
            System.out.println("ID : "+id);
            System.out.println("GMAIL : "+gmail);
            System.out.println("YAHOO : "+yahoo);

            System.out.println();
            rs.next();
            System.out.println("Displaying records for next row");

            id = rs.getInt("id");
            gmail = rs.getString("gmail");
            yahoo = rs.getString("yahoo");

            //Displaying records in last row
            System.out.println("ID : "+id);
            System.out.println("GMAIL : "+gmail);
            System.out.println("YAHOO : "+yahoo);

        }catch(SQLException e){

            e.printStackTrace();

        }catch(ClassNotFoundException e){

             System.out.println("Plese check the driver class path "+e.getMessage());

        }finally{

            if(stmt != null){

                stmt.close();

            }
            if(dbConnection != null){

                dbConnection.close();

            }
        }
    }
}

输出

显示最后一行的记录 ID：5 GMAIL：naveen 雅虎：naveenrockz

显示第一行的记录 ID：1 GMAIL：vinothvino 雅虎：vinothasd

显示下一行的记录 ID：2 GMAIL：ajithvirje 雅虎：ajith234

请让我理解。为什么我的代码不能获取CODE SAMPLE 1程序中的任何记录？

下图代表emp4表中的以下记录

Answer 1

原因是在使用ResultSet / getInt()访问数据之前，您未通过调用next()来推进getString()的光标。尝试这样的事情：

//Creating ResultSet
ResultSet rs = stmt.executeQuery(sql);

//Displaying database
System.out.println("Displaying records before doing some operations");

if (rs.next()) {        
    System.out.println(rs.getInt(1) + " "
        + rs.getString(2) + " " + rs.getString(3));
}

如果要遍历整个结果集，请改用while (rs.next())。

您的第二个代码段有效，因为您在第一次访问列值之前将光标移动到rs.last()的最后位置。

请注意，在访问列值之前，您应始终检查rs.next() / rs.last() / rs.first()方法的返回值。 falsey返回值表示结果集没有行，并且在调用结果集的任何getter（rs.getInt() / rs.getString()等）方法时会引发异常。

Answer 2

你需要使用.next（）函数跳转到结果集中的第一条记录

while(rs.next()){
        System.out.println(rs.getInt(1)+" "+rs.getString(2)+" "+rs.getString(3));        }

在JDBC

2 个答案: