目录摘要：一、子查询二、连接查询三、聚合函数四、GROUP BY 和 HAVING五、窗口函数六、CASE 语句七、事务处理八、自定义排序（ORDER BY FIELD）九、空值NULL排序(ORDER BY IF(ISNULL))十、CASE表达式(CASE·&
the situation is like this: im creating a Logger class that can write to a file but the write_to_file() function is in a helper class as a static function.
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