Hvordan kan jeg få procentdelen af ​​de samlede rækker med mysql for en gruppe?

Ja, du kan:

select num, string, 100 * num / total as percent
from (
    select count(*) as num, string
    from useragent_ip
    left join useragents on useragent_id = useragents.id
    group by useragent_id) x
cross join (
    select count(*) as total
    from useragent_ip
    left join useragents on useragent_id = useragents.id) y
order by num desc, string;

Jeg fjernede having num > 2 , fordi det så ikke ud til at give mening.