leetcodeSQL:602. Friend application II: who has the most friends

Ideas ：
Turn the watch upside down union once
then groupby count Just fine

sql

# Write your MySQL query statement belowse
select l1 as id, count(l1) as num
from
(
    select requester_id as l1, accepter_id as l2
    from RequestAccepted
    Union #  Remove duplication 
    select accepter_id as l1, requester_id as l2
    from RequestAccepted
) t
group by l1
order by num DESC
limit 1