参考文章:
https://maoyifa100.iteye.com/blog/1900305
SET @xx="name,bm";
SET @sql1 = CONCAT('select ',@xx,' from people');
prepare s1 from @sql1; # 预定义一个语句
execute s1; # 执行预定义语句
deallocate prepare s1; # 释放一个预定义语句的资源
预编译语句作用域为本次连接客户端
连接断开资源自动释放