本章提供了有关如何使用JDBC应用程序从表中选择记录的示例.这将在从表中选择记录时使用LIKE子句添加其他条件.在执行以下示例之前，请确保您具有以下内容和减号;
要执行以下示例，您可以替换用户名和密码包含您的实际用户名和密码.
您的MySQL或任何数据库正在使用，正在运行.
以下步骤是必需的使用JDBC应用程序创建一个新数据库 :
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，以便打开与数据库的通信通道.
打开连接:需要使用 DriverManager.getConnection()方法，用于创建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，以从满足给定条件的表中获取记录.此查询使用 LIKE 子句选择记录，以选择名字以"za"开头的所有学生.
清理环境:需要显式关闭所有数据库资源，而不是依赖于JVM的垃圾回收.
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 without any condition.
System.out.println("Fetching records without condition...");
String sql = "SELECT id, first, last, age FROM Registration";
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 Registration" +
" WHERE first LIKE '%za%' ";
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，让我们编译上面的例子如下 :
C:\> javac JDBCExample.java C:\>
运行 JDBCExample 时，会产生以下结果 :
C:\>java JDBCExample Connecting to a selected database... Connected database successfully... Creating statement... Fetching records without condition... ID: 100, Age: 30, First: Zara, Last: Ali ID: 102, Age: 30, First: Zaid, Last: Khan ID: 103, Age: 28, First: Sumit, Last: Mittal Fetching records with condition... ID: 100, Age: 30, First: Zara, Last: Ali ID: 102, Age: 30, First: Zaid, Last: Khan Goodbye! C:\>
