📜  JDBC-排序数据示例

📅  最后修改于: 2020-11-13 04:42:22             🧑  作者: Mango


本章提供了有关如何使用JDBC应用程序对表中的记录进行排序的示例。这将使用ascdesc关键字对记录进行升序或降序排序。在执行以下示例之前,请确保已具备以下条件:

  • 要执行以下示例,您可以将用户名密码替换为实际的用户名和密码。

  • 您的MySQL或您正在使用的任何数据库均已启动并正在运行。

必要步骤

使用JDBC应用程序创建新数据库需要执行以下步骤-

  • 导入软件包:要求您包含包含数据库编程所需的JDBC类的软件包。通常,使用import java.sql。*就足够了。

  • 注册JDBC驱动程序:要求您初始化驱动程序,以便您可以打开与数据库的通信通道。

  • 打开连接:需要使用DriverManager.getConnection()方法创建一个Connection对象,该对象表示与数据库服务器的物理连接。

  • 执行查询:需要使用Statement类型的对象来构建和提交SQL语句以对表中的记录进行排序。这些查询使用ascdesc子句对数据进行升序和降序排序。

  • 清理环境:需要显式关闭所有数据库资源,而不是依赖JVM的垃圾回收。

样例代码

将以下示例复制并粘贴到JDBCExample.java中,编译并运行如下-

//STEP 1. Import required packages
import java.sql.*;

public class JDBCExample {
   // JDBC driver name and database URL
   static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";  
   static final String DB_URL = "jdbc:mysql://localhost/STUDENTS";

   //  Database credentials
   static final String USER = "username";
   static final String PASS = "password";
   
   public static void main(String[] args) {
   Connection conn = null;
   Statement stmt = null;
   try{
      //STEP 2: Register JDBC driver
      Class.forName("com.mysql.jdbc.Driver");

      //STEP 3: Open a connection
      System.out.println("Connecting to a selected database...");
      conn = DriverManager.getConnection(DB_URL, USER, PASS);
      System.out.println("Connected database successfully...");
      
      //STEP 4: Execute a query
      System.out.println("Creating statement...");
      stmt = conn.createStatement();

      // Extract records in ascending order by first name.
      System.out.println("Fetching records in ascending order...");
      String sql = "SELECT id, first, last, age FROM Registration" +
                   " ORDER BY first ASC";
      ResultSet rs = stmt.executeQuery(sql);

      while(rs.next()){
         //Retrieve by column name
         int id  = rs.getInt("id");
         int age = rs.getInt("age");
         String first = rs.getString("first");
         String last = rs.getString("last");

         //Display values
         System.out.print("ID: " + id);
         System.out.print(", Age: " + age);
         System.out.print(", First: " + first);
         System.out.println(", Last: " + last);
      }

      // Extract records in descending order by first name.
      System.out.println("Fetching records in descending order...");
      sql = "SELECT id, first, last, age FROM Registration" +
                   " ORDER BY first DESC";
      rs = stmt.executeQuery(sql);

      while(rs.next()){
         //Retrieve by column name
         int id  = rs.getInt("id");
         int age = rs.getInt("age");
         String first = rs.getString("first");
         String last = rs.getString("last");

         //Display values
         System.out.print("ID: " + id);
         System.out.print(", Age: " + age);
         System.out.print(", First: " + first);
         System.out.println(", Last: " + last);
      }
      rs.close();
   }catch(SQLException se){
      //Handle errors for JDBC
      se.printStackTrace();
   }catch(Exception e){
      //Handle errors for Class.forName
      e.printStackTrace();
   }finally{
      //finally block used to close resources
      try{
         if(stmt!=null)
            conn.close();
      }catch(SQLException se){
      }// do nothing
      try{
         if(conn!=null)
            conn.close();
      }catch(SQLException se){
         se.printStackTrace();
      }//end finally try
   }//end try
   System.out.println("Goodbye!");
}//end main
}//end JDBCExample

现在,让我们编译上面的示例,如下所示:

C:\>javac JDBCExample.java
C:\>

当您运行JDBCExample时,它将产生以下结果-

C:\>java JDBCExample
Connecting to a selected database...
Connected database successfully...
Creating statement...
Fetching records in ascending order...
ID: 103, Age: 28, First: Sumit, Last: Mittal
ID: 102, Age: 30, First: Zaid, Last: Khan
ID: 100, Age: 30, First: Zara, Last: Ali
Fetching records in descending order...
ID: 100, Age: 30, First: Zara, Last: Ali
ID: 102, Age: 30, First: Zaid, Last: Khan
ID: 103, Age: 28, First: Sumit, Last: Mittal
Goodbye!
C:\>