_1369.sql

--# Write your MySQL query statement below
--credit: https://leetcode.com/problems/get-the-second-most-recent-activity/discuss/530992/MySQL-solution-184ms-no-subquery

select * from UserActivity
group by username
having count(*) = 1

union all

select u1.*
from UserActivity as u1
left join UserActivity as u2
on u1.username = u2.username and u1.endDate < u2.endDate
group by u1.username, u1.endDate
having count(u2.endDate) = 1