SQL practice #5

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.

My solution
Db-fiddle for this problem

The key intuition is to use 'union all' to merge the requester_id and accepter_id columns.

SELECT a.id, count(a.id) num
FROM
(
(SELECT requester_id id FROM request_accepted)
UNION ALL
(SELECT accepter_id id FROM request_accepted)
) a
group by a.id
order by count(a.id) DESC
limit 1

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