Java Mysql 简明教程

Java & MySQL - WHERE Clause Example

本章提供了一个使用 JDBC 应用程序从表中选择记录的示例。它会添加附加条件，同时使用 WHERE 子句从表中选择记录。在执行以下示例之前，请确保已准备好以下内容 -

This chapter provides an example on how to select records from a table using JDBC application. This would add additional conditions using WHERE clause while selecting records from the table. Before executing the following example, make sure you have the following in place −

To execute the following example you can replace the username and password with your actual user name and password.
Your MySQL database you are using, is up and running.

Required Steps

创建新的数据库使用 JDBC 应用程序需要执行以下步骤：

The following steps are required to create a new Database using JDBC application −

Import the packages − Requires that you include the packages containing the JDBC classes needed for the database programming. Most often, using import java.sql.* will suffice.
Register the JDBC driver − Requires that you initialize a driver so you can open a communications channel with the database.
Open a connection − Requires using the DriverManager.getConnection() method to create a Connection object, which represents a physical connection with a database server.
Execute a query − Requires using an object of type Statement for building and submitting an SQL statement to fetch records from a table, which meet the given condition. This Query makes use of the WHERE clause to select records.
Clean up the environment − try with resources automatically closes the resources.

Sample Code

将以下示例复制并粘贴到 TestApplication.java 中，如下进行编译和运行：

Copy and paste the following example in TestApplication.java, compile and run as follows −

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class TestApplication {
   static final String DB_URL = "jdbc:mysql://localhost/TUTORIALSPOINT";
   static final String USER = "guest";
   static final String PASS = "guest123";
   static final String QUERY = "SELECT id, first, last, age FROM Registration";

   public static void main(String[] args) {
      // Open a connection
      try(Connection conn = DriverManager.getConnection(DB_URL, USER, PASS);
         Statement stmt = conn.createStatement();) {
         System.out.println("Fetching records without condition...");
         ResultSet rs = stmt.executeQuery(QUERY);
         while(rs.next()){
            //Display values
            System.out.print("ID: " + rs.getInt("id"));
            System.out.print(", Age: " + rs.getInt("age"));
            System.out.print(", First: " + rs.getString("first"));
            System.out.println(", Last: " + rs.getString("last"));
         }

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

         while(rs.next()){
            //Display values
            System.out.print("ID: " + rs.getInt("id"));
            System.out.print(", Age: " + rs.getInt("age"));
            System.out.print(", First: " + rs.getString("first"));
            System.out.println(", Last: " + rs.getString("last"));
         }
         rs.close();
      } catch (SQLException e) {
         e.printStackTrace();
      }
   }
}

现在让我们如下编译上述示例：

Now let us compile the above example as follows −

C:\>javac TestApplication.java
C:\>

运行 TestApplication 时，它会生成以下结果：

When you run TestApplication, it produces the following result −

C:\>java TestApplication
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: 102, Age: 30, First: Zaid, Last: Khan
ID: 103, Age: 28, First: Sumit, Last: Mittal
C:\>