JDBC实例-WHERE子句

在本教程将演示如何在JDBC应用程序中，从数据库表中查询数据记录，在查询选择记录时使用WHERE子句添加其他条件。在执行以下示例之前，请确保您已经准备好以下操作：

具有数据库管理员权限，以在给定模式的数据库表中查询数据记录。要执行以下示例，需要用实际用户名和密码替换这里用户名(username)和密码(password)。
MySQL或数据库已启动并运行。

所需步骤

使用JDBC应用程序查询表中数据记录需要以下步骤：

导入包：需要包含包含数据库编程所需的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：需要初始化驱动程序，以便可以程序中打开数据库的通信通道。
打开连接：需要使用DriverManager.getConnection()方法来创建一个Connection对象，它表示与数据库服务器的物理连接。
执行查询：需要使用类型为Statement的对象来构建和提交SQL语句，以从表中获取满足给定条件的记录。查询语句使用WHERE子句来选择记录。
清理环境：需要明确地关闭所有数据库资源，而不依赖于JVM的垃圾收集。

示例代码

复制以下示例代码保存到文件：SelectWhereClause.java中，然后编译并运行如下:

//STEP 1. Import required packages

import java.sql.*;

public class SelectWhereClause {
   // 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();

      // Extract records without any condition.
      System.out.println("Fetching records without condition...");
      String sql = "SELECT id, first, last, age FROM student";
      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);
      }

      // Select all records having ID equal or greater than 101
      System.out.println("Fetching records with condition...");
      sql = "SELECT id, first, last, age FROM student" +
                   " WHERE id >= 103 ";
      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

运行结果

Connecting to a selected database...
Connected database successfully...
Creating statement...
Fetching records without condition...
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
Fetching records with condition...
ID: 103, Age: 28, First: Java, Last: Ja
Goodbye!