Cannot drop table 'xxx' referenc

执行DROP TABLE时可能会偶遇referenced by a foreign key constraint的问题，完整的错误信息如下：

Cannot drop table 'tb_app' referenced by a foreign key constraint 
'FK7gwn5l5wkgc0kl6jqwscyev3c' on table 'tb_package'

当遇到这种情况时应该先关闭MySQL的FOREIGN_KEY_CHECKS，详细命令如下：

// 关闭MySQL的`FOREIGN_KEY_CHECKS`
FOREIGN_KEY_CHECKS=0;

// 删除表数据
DROP TABLE xxxx;

// 重新开启MySQL的`FOREIGN_KEY_CHECKS`
SET FOREIGN_KEY_CHECKS=1;