SELECT user_id, COUNT(*) AS nr_of_logins
FROM table
GROUP BY user_id
HAVING nr_of_logins = 1

SELECT user_id, COUNT(*) AS nr_of_logins
FROM table
GROUP BY user_id
HAVING nr_of_logins > 1

select user_id, count(*)
from table
group by user_id
having count(*)=1

select user_id, count(*)
from table
group by user_id
having count(*)>1