Friend Requests II: Who Has the Most Friends.py

'''
In social network like Facebook or Twitter, people send friend requests and accept others' requests as well.

Table request_accepted holds the data of friend acceptance, while requester_id and accepter_id both are the id of a person.

| requester_id | accepter_id | accept_date|
|--------------|-------------|------------|
| 1            | 2           | 2016_06-03 |
| 1            | 3           | 2016-06-08 |
| 2            | 3           | 2016-06-08 |
| 3            | 4           | 2016-06-09 |

Write a query to find the the people who has most friends and the most friends number. For the sample data above, the result is:

| id | num |
|----|-----|
| 3  | 3   |

Note:
It is guaranteed there is only 1 people having the most friends.
The friend request could only been accepted once, which mean there is no multiple records with the same requester_id and accepter_id value.

Explanation:
The person with id '3' is a friend of people '1', '2' and '4', so he has 3 friends in total, which is the most number than any others.

Follow-up:
In the real world, multiple people could have the same most number of friends, can you find all these people in this case? 
'''

# Write your MySQL query statement below
select first_id as id, count(second_id) as num
from (
select requester_id as first_id, accepter_id as second_id
    from request_accepted
union all
select accepter_id as first_id, requester_id as second_id
    from request_accepted
) t
group by first_id
order by num desc
limit 1