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JDBC查詢數據實例

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在本教程將演示如何在JDBC應用程序中,查詢數據庫的一個表中數據記錄。 在執行以下示例之前,請確保您已經準備好以下操作:

  • 具有數據庫管理員權限,以在給定模式中數據庫表中查詢數據記錄。 要執行以下示例,需要用實際用戶名和密碼替換這裏用戶名(username)和密碼(password)。
  • MySQL或數據庫已啟動並運行。

所需步驟

使用JDBC應用程序向表中查詢數據記錄需要以下步驟:

  • 導入包:需要包含包含數據庫編程所需的JDBC類的包。 大多數情況下,使用import java.sql.*就足夠了。

  • 註冊JDBC驅動程序:需要初始化驅動程序,以便可以程序中打開數據庫的通信通道。

  • 打開連接

    :需要使用DriverManager.getConnection()方法來創建一個Connection對象,它表示與數據庫服務器的物理連接。

  • 執行查詢:需要使用類型為Statement的對象來構建和提交SQL語句,以在選擇的數據庫的表中查詢數據記錄。

  • 提取數據:執行SQL查詢後,可以從表中獲取記錄。

  • 清理環境:需要明確地關閉所有數據庫資源,而不依賴於JVM的垃圾收集。

示例代碼

復制以下示例代碼保存到文件:SelectRecords.java中,然後編譯並運行如下 -

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

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

   //  Database credentials
   static final String USER = "root";
   static final String PASS = "123456";

   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();

      String sql = "SELECT id, first, last, age FROM student";
      ResultSet rs = stmt.executeQuery(sql);
      //STEP 5: Extract data from result set
      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
Java

編譯上面代碼,如下 -

F:\worksp\jdbc> javac -Djava.ext.dirs=F:\worksp\jdbc\libs SelectRecords.java
Shell

執行上面代碼,如下 -

## 可先創建表或插入數據 -
## F:\worksp\jdbc> java -Djava.ext.dirs=F:\worksp\jdbc\libs InsertRecords

F:\worksp\jdbc>java -Djava.ext.dirs=F:\worksp\jdbc\libs SelectRecords
Connecting to a selected database...
Thu Jun 01 23:11:55 CST 2017 WARN: Establishing SSL connection without server‘s identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn‘t set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to ‘false‘. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.
Connected database successfully...
Creating statement...
ID: 100, Age: 18, First: C++, Last: Li
ID: 101, Age: 25, First: Python, Last: Py
ID: 102, Age: 30, First: Ruby, Last: Ru
ID: 103, Age: 28, First: Java, Last: Ja
Goodbye!

F:\worksp\jdbc>

JDBC查詢數據實例