查看 JDBC 中的结果集时不检索记录

Question

现在我正在学习 java 中的结果集类型。在这里，我编写了以不同方式查看记录的代码。起初我显示了 emp4 table 中的全部记录然后我开始以不同的方式查看这些记录（最后，第一，下一个）这正是我正在寻找的但它不会显示所有emp4 table 中显示的记录。请参阅第一个程序，它不起作用，但如果我记录了第 41 行（请参阅第二个程序中的这个），它就可以正常工作。有什么问题？我的代码有什么问题吗？？？

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，代码可以正常工作......

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编号：5 GMAIL : 纳文雅虎：naveenrockz

显示第一行的记录编号：1 GMAIL : 葡萄酒雅虎 : vinothasd

显示下一行的记录编号：2 GMAIL : 邮件雅虎：ajith234

请让我明白。为什么我的代码没有获取 CODE SAMPLE 1 程序中的任何记录？

下图代表emp4中的以下记录table

Answer 1

原因是在使用getInt() / getString() 访问数据之前，您没有通过调用next() 来推进ResultSet 的游标。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())。

您的第二个代码片段有效，因为您在第一次访问列值之前使用 rs.last() 将光标移动到最后一个位置。

请注意，在访问列值之前，您应该始终检查 rs.next() / rs.last() / rs.first() 方法的 return 值。错误的 return 值表示结果集没有行，并且会导致在调用任何 getter（rs.getInt() / rs.getString() 等）方法时抛出异常结果集的数量。

Answer 2

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));        }

查看 JDBC 中的结果集时不检索记录

Doesn't retrieve the records while viewing a ResultSet in JDBC

java

mysql

jdbc

resultset

data-retrieval