SQL注入是一种常见的网络攻击手段，攻击者通过在Web应用中输入恶意SQL代码，实现对数据库的非法访问或篡改。Union型SQL注入是其中一种高级技巧，本文将深入剖析其原理、技巧及防范措施。
一、Union型SQL注入原理
Union型SQL注入是利用SQL的UNION关键字实现攻击的一种方式。UNION关键字可以将多个SELECT语句的结果集合并为一个结果集。攻击者通过构造特殊的SQL语句，使得原本应该返回单个结果集的查询，在执行时返回两个或多个结果集。
例如，假设存在一个用户登录功能，其SQL语句如下：
SELECT username, password FROM users WHERE username = 'admin' AND password = '123456';
攻击者可以构造如下SQL语句：
”`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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