26、JDBC 指南 - 排序实例

2019-07-03

本章介绍了如何使用 JDBC 应用程序将表中记录进行排序的示例。该示例将使用 ascdesc 作为关键字将记录按升序或降序排序。执行下面的示例之前,请确保你已做好以下工作-

  • 在运行下面的例子之前,你需要用你实际的用户名和密码去代替 usernamepassword
  • 你的 MySQL 或者其他数据库已经启动了并在运行中。

所需的步骤

用 JDBC 应用程序去创建一个新的数据库需要执行以下步骤-

  • 导入包:要求你包括含有需要进行数据库编程的 JDBC 类的包。大多数情况下,使用 import java.sql. 就足够了。

  • 注册 JDBC 驱动程序:要求你初始化驱动程序,这样你可以与数据库打开通信通道。
  • 打开连接:需要使用 DriverManager.getConnection() 方法创建一个 Connection 对象,它代表与数据库服务器的物理连接。
  • 执行查询:需要使用类型声明的对象建立并提交一个 SQL 语句,并依据提供的条件将表的记录进行排序。这个查询语句用 ascdesc 子句对数据进行升序或者降序排序。
  • 清理环境:依靠 JVM 垃圾收集器可以明确地回收所有的数据库资源。

示例代码

将下面的示例拷贝并粘帖到 JDBCExample.java 中,编译并运行它,如下所示-

//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,让我们用下面的命令编译上面的代码-

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

当你运行 JDBCExample 时,它将展示下面的结果-

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:\>