引言
SQL注入是一种常见的网络攻击手段，攻击者通过在Web应用程序中插入恶意SQL代码，从而获取、修改或删除数据库中的数据。本文将深入探讨如何利用联合查询和ORDER BY子句进行SQL注入攻击，帮助读者了解这一攻击手段的原理和防范措施。
联合查询（UNION）
联合查询是一种将两个或多个SELECT语句的结果集合并成一个结果集的查询方式。攻击者可以利用联合查询来绕过数据库的安全限制，从而获取敏感信息。
原理
假设存在一个登录页面，其SQL查询语句如下：
SELECT username, password FROM users WHERE username = '$username' AND password = '$password';
攻击者可以通过以下方式构造恶意输入：
”`sql ‘ OR ‘1’=‘1’ UNION SELECT NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, 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