602_Friend Requests II: Who Has the Most Friends

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

Table request_acceptedholds 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?

Solution

SELECT id, COUNT(id) AS num
FROM 
    (
    SELECT requester_id AS id
    FROM request_accepted
    UNION ALL
    SELECT accepter_id AS id
    FROM request_accepted    
    ) AS totoal_table
GROUP BY id
ORDER BY COUNT(id) DESC
LIMIT 1;