Leetcode1501.可以放心投资的国家（困难）

题目
Table Person:

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| id             | int     |
| name           | varchar |
| phone_number   | varchar |
+----------------+---------+

id is the primary key for this table.
Each row of this table contains the name of a person and their phone number.
Phone number will be in the form 'xxx-yyyyyyy' where xxx is the country code (3 characters) and yyyyyyy is the phone number (7 characters) where x and y are digits. Both can contain leading zeros.
Table Country:

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| name           | varchar |
| country_code   | varchar |
+----------------+---------+

country_code is the primary key for this table.
Each row of this table contains the country name and its code. country_code will be in the form 'xxx' where x is digits.

Table Calls:

+-------------+------+
| Column Name | Type |
+-------------+------+
| caller_id   | int  |
| callee_id   | int  |
| duration    | int  |
+-------------+------+

There is no primary key for this table, it may contain duplicates.
Each row of this table contains the caller id, callee id and the duration of the call in minutes. caller_id != callee_id
A telecommunications company wants to invest in new countries. The country intends to invest in the countries where the average call duration of the calls in this country is strictly greater than the global average call duration.

Write an SQL query to find the countries where this company can invest.

Return the result table in any order.

The query result format is in the following example.

Person table:

+----+----------+--------------+
| 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 table:

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

Calls table:

+-----------+-----------+----------+
| 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 table:

+----------+
| country  |
+----------+
| Peru     |
+----------+

The average call duration for Peru is (102 + 102 + 330 + 330 + 5 + 5) / 6 = 145.666667
The average call duration for Israel is (33 + 4 + 13 + 13 + 3 + 1 + 1 + 7) / 8 = 9.37500
The average call duration for Morocco is (33 + 4 + 59 + 59 + 3 + 7) / 6 = 27.5000
Global call duration average = (2 * (33 + 3 + 59 + 102 + 330 + 5 + 13 + 3 + 1 + 7)) / 20 = 55.70000
Since Peru is the only country where average call duration is greater than the global average, it's the only recommended country

生成数据

解答
Person的phone_number的前三位可以得到对应的国家
将calls 拆分为两个 union 再和Person连接按country_code分组统计均值
然后和总体的均值比较即可

先拆分为两个union

SELECT C.`callee_id` AS id, C.`duration`
FROM Calls AS C
UNION ALL
SELECT CC.`caller_id` AS id, CC.`duration`
FROM Calls AS CC;

Person的phone_number的前三位可以得到对应的国家

SELECT P.`id`, P.`name`, SUBSTRING(P.`phone_number`,1,3) AS country_code
FROM Person3 AS P;

两表连接按country_code分组统计均值

SELECT B.country_code, AVG(A.duration) AS avg_dur
FROM (SELECT C.`callee_id` AS id, C.`duration`
FROM Calls AS C
UNION ALL
SELECT CC.`caller_id` AS id, CC.`duration`
FROM Calls AS CC) AS A
LEFT JOIN (SELECT P.`id`, P.`name`, SUBSTRING(P.`phone_number`,1,3) AS country_code
FROM Person3 AS P) AS B
ON A.id = B.id
GROUP BY B.country_code;

选出超出整体平均值的country_code

SELECT B.country_code
FROM (SELECT C.`callee_id` AS id, C.`duration`
FROM Calls AS C
UNION ALL
SELECT CC.`caller_id` AS id, CC.`duration`
FROM Calls AS CC) AS A
LEFT JOIN (SELECT P.`id`, P.`name`, SUBSTRING(P.`phone_number`,1,3) AS country_code
FROM Person3 AS P) AS B
ON A.id = B.id
GROUP BY B.country_code
HAVING AVG(A.duration) >= (SELECT AVG(CCC.`duration`)
FROM Calls AS CCC);

与country连接得到对应name

SELECT Co.`name`
FROM (SELECT B.country_code
FROM (SELECT C.`callee_id` AS id, C.`duration`
FROM Calls AS C
UNION ALL
SELECT CC.`caller_id` AS id, CC.`duration`
FROM Calls AS CC) AS A
LEFT JOIN (SELECT P.`id`, P.`name`, SUBSTRING(P.`phone_number`,1,3) AS country_code
FROM Person3 AS P) AS B
ON A.id = B.id
GROUP BY B.country_code
HAVING AVG(A.duration) >= (SELECT AVG(CCC.`duration`)
FROM Calls AS CCC)) AS D
JOIN Country AS Co
ON Co.`country_code` = D.country_code;