leetcode-SQL-1699. 两人之间的通话次数

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| from_id     | int     |
| to_id       | int     |
| duration    | int     |
+-------------+---------+
该表没有主键，可能存在重复项。
该表包含 from_id 与 to_id 间的一次电话的时长。
from_id != to_id

输入：
Calls 表：
+---------+-------+----------+
| from_id | to_id | duration |
+---------+-------+----------+
| 1       | 2     | 59       |
| 2       | 1     | 11       |
| 1       | 3     | 20       |
| 3       | 4     | 100      |
| 3       | 4     | 200      |
| 3       | 4     | 200      |
| 4       | 3     | 499      |
+---------+-------+----------+
输出：
+---------+---------+------------+----------------+
| person1 | person2 | call_count | total_duration |
+---------+---------+------------+----------------+
| 1       | 2       | 2          | 70             |
| 1       | 3       | 1          | 20             |
| 3       | 4       | 4          | 999            |
+---------+---------+------------+----------------+
解释：
用户 1 和 2 打过 2 次电话，总时长为 70 (59 + 11)。
用户 1 和 3 打过 1 次电话，总时长为 20。
用户 3 和 4 打过 4 次电话，总时长为 999 (100 + 200 + 200 + 499)。

select
    person1,
    person2,
    count(*) as call_count,
    sum(duration) as total_duration
from(
    select 
        if(from_id>to_id,to_id,from_id) as person1,
        if(from_id>to_id,from_id,to_id) as person2,
        duration
    from Calls
)as c
group by person1,person2

select
    from_id as person1,
    to_id as person2,
    count(*) as call_count,
    sum(duration) as total_duration
from Calls
group by least(from_id,to_id),greatest(from_id,to_id);