So, why do you store multiple values in a column? It goes against the grain of all sound database design and makes quering very complicated.
create an additional table with two columns, one to identify a user, one for identifying the country. Store each combination of user and country as separate records.
To get all countries a user has visited, use the following query
select username,countryName
from users u left join countriesVisited cv
on u.userid = cv.userid
left join countries c
on cv.countryid = c.countryid
I did not follow your naming standard as I thought it counterintuitive.