我如何获得一个组的mysql总行数的百分比？

是的你可以：

select num, string, 100 * num / total as percentfrom (    select count(*) as num, string    from useragent_ip    left join useragents on useragent_id = useragents.id    group by useragent_id) xcross join (    select count(*) as total    from useragent_ip    left join useragents on useragent_id = useragents.id) yorder by num desc, string;

我删除了

having num > 2