「SQL面试题库」 No_117 可以放心投资的国家

今日真题

题目介绍： 可以放心投资的国家 countries-you-can-safely-invest-in

难度中等

SQL架构

表

Person

:

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| id             | int     |
| name           | varchar |
| phone_number   | varchar |
+----------------+---------+
id 是该表主键.
该表每一行包含一个人的名字和电话号码.
电话号码的格式是:'xxx-yyyyyyy', 其中xxx是国家码(3个字符), yyyyyyy是电话号码(7个字符), x和y都表示数字. 同时, 国家码和电话号码都可以包含前导0.

表

Country

:

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| name           | varchar |
| country_code   | varchar |
+----------------+---------+
country_code是该表主键.
该表每一行包含国家名和国家码. country_code的格式是'xxx', x是数字.

表

Calls

:

+-------------+------+
| Column Name | Type |
+-------------+------+
| caller_id   | int  |
| callee_id   | int  |
| duration    | int  |
+-------------+------+
该表无主键, 可能包含重复行.
每一行包含呼叫方id, 被呼叫方id和以分钟为单位的通话时长. caller_id != callee_id

一家电信公司想要投资新的国家. 该公司想要投资的国家是: 该国的平均通话时长要严格地大于全球平均通话时长.

写一段 SQL, 找到所有该公司可以投资的国家.

返回的结果表没有顺序要求.

查询的结果格式如下例所示.

``` Person 表: +----+----------+--------------+ | id | name | phone_number | +----+----------+--------------+ | 3 | Jonathan | 051-1234567 | | 12 | Elvis | 051-7654321 | | 1 | Moncef | 212-1234567 | | 2 | Maroua | 212-6523651 | | 7 | Meir | 972-1234567 | | 9 | Rachel | 972-0011100 | +----+----------+--------------+

Country 表: +----------+--------------+ | name | country_code | +----------+--------------+ | Peru | 051 | | Israel | 972 | | Morocco | 212 | | Germany | 049 | | Ethiopia | 251 | +----------+--------------+

Calls 表: +-----------+-----------+----------+ | caller_id | callee_id | duration | +-----------+-----------+----------+ | 1 | 9 | 33 | | 2 | 9 | 4 | | 1 | 2 | 59 | | 3 | 12 | 102 | | 3 | 12 | 330 | | 12 | 3 | 5 | | 7 | 9 | 13 | | 7 | 1 | 3 | | 9 | 7 | 1 | | 1 | 7 | 7 | +-----------+-----------+----------+

Result 表: +----------+ | country | +----------+ | Peru | +----------+ 国家Peru的平均通话时长是 (102 + 102 + 330 + 330 + 5 + 5) / 6 = 145.666667 国家Israel的平均通话时长是 (33 + 4 + 13 + 13 + 3 + 1 + 1 + 7) / 8 = 9.37500 国家Morocco的平均通话时长是 (33 + 4 + 59 + 59 + 3 + 7) / 6 = 27.5000 全球平均通话时长 = (2 * (33 + 3 + 59 + 102 + 330 + 5 + 13 + 3 + 1 + 7)) / 20 = 55.70000 所以, Peru是唯一的平均通话时长大于全球平均通话时长的国家, 也是唯一的推荐投资的国家. ```

笛卡尔积

sql
select c2.name as country 
from Calls c1,Person p,Country c2
where (p.id=c1.caller_id or p.id=c1.callee_id) and c2.country_code=left(p.phone_number,3)
group by c2.name 
having avg(duration)>(select avg(duration) from Calls)

思路更清晰

```sql with people_country as ( select id, c.name country from Person p left join Country c on left(p.phone_number,3) = c.country_code )

select country from ( select country, avg(duration) avgtime from ( select caller_id id, duration from Calls union all select callee_id, duration from Calls ) t left join people_country using(id) group by country ) temp where avgtime > ( select avg(duration) avgtime from ( select caller_id, duration from Calls union all select callee_id, duration from Calls ) t ) ```

• 已经有灵感了？在评论区写下你的思路吧！