JDBC 排序?qū)嵗?/h1>

排序?qū)嵗?/h2>

本章介紹了如何使用 JDBC 應(yīng)用程序?qū)⒈碇杏涗涍M(jìn)行排序的示例。該示例將使用 asc 和 desc 作為關(guān)鍵字將記錄按升序或降序排序。執(zhí)行下面的示例之前，請確保你已做好以下工作-

• 在運(yùn)行下面的例子之前，你需要用你實(shí)際的用戶名和密碼去代替 username 和 password。
• 你的 MySQL 或者其他數(shù)據(jù)庫已經(jīng)啟動了并在運(yùn)行中。

所需的步驟

用 JDBC 應(yīng)用程序去創(chuàng)建一個新的數(shù)據(jù)庫需要執(zhí)行以下步驟-

• 導(dǎo)入包：要求你包括含有需要進(jìn)行數(shù)據(jù)庫編程的 JDBC 類的包。大多數(shù)情況下，使用 import java.sql. 就足夠了。

• 注冊 JDBC 驅(qū)動程序：要求你初始化驅(qū)動程序，這樣你可以與數(shù)據(jù)庫打開通信通道。
• 打開連接：需要使用 DriverManager.getConnection() 方法創(chuàng)建一個 Connection 對象，它代表與數(shù)據(jù)庫服務(wù)器的物理連接。
• 執(zhí)行查詢：需要使用類型聲明的對象建立并提交一個 SQL 語句，并依據(jù)提供的條件將表的記錄進(jìn)行排序。這個查詢語句用 asc 和 desc 子句對數(shù)據(jù)進(jìn)行升序或者降序排序。
• 清理環(huán)境：依靠 JVM 垃圾收集器可以明確地回收所有的數(shù)據(jù)庫資源。

示例代碼

將下面的示例拷貝并粘帖到 JDBCExample.java 中，編譯并運(yùn)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

現(xiàn)在，讓我們用下面的命令編譯上面的代碼-

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

當(dāng)你運(yùn)行 JDBCExample 時(shí)，它將展示下面的結(jié)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:\>