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 company 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.
Example 1:
Input: 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 |
Output:
| country |
|---|
| Peru |
Explanation: 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 + 4 + 59 + 102 + 330 + 5 + 13 + 3 + 1 + 7)) / 20 = 55.70000
Since Peru is the only country where the average call duration is greater than the global average, it is the only recommended country.
Data
Create table If Not Exists Person
(
id int,
name varchar(15),
phone_number varchar(11)
);
Create table If Not Exists Country
(
name varchar(15),
country_code varchar(3)
);
Create table If Not Exists Calls
(
caller_id int,
callee_id int,
duration int
);
Truncate table Person;
insert into Person (id, name, phone_number)
values ('3', 'Jonathan', '051-1234567');
insert into Person (id, name, phone_number)
values ('12', 'Elvis', '051-7654321');
insert into Person (id, name, phone_number)
values ('1', 'Moncef', '212-1234567');
insert into Person (id, name, phone_number)
values ('2', 'Maroua', '212-6523651');
insert into Person (id, name, phone_number)
values ('7', 'Meir', '972-1234567');
insert into Person (id, name, phone_number)
values ('9', 'Rachel', '972-0011100');
Truncate table Country;
insert into Country (name, country_code)
values ('Peru', '051');
insert into Country (name, country_code)
values ('Israel', '972');
insert into Country (name, country_code)
values ('Morocco', '212');
insert into Country (name, country_code)
values ('Germany', '049');
insert into Country (name, country_code)
values ('Ethiopia', '251');
Truncate table Calls;
insert into Calls (caller_id, callee_id, duration)
values ('1', '9', '33');
insert into Calls (caller_id, callee_id, duration)
values ('2', '9', '4');
insert into Calls (caller_id, callee_id, duration)
values ('1', '2', '59');
insert into Calls (caller_id, callee_id, duration)
values ('3', '12', '102');
insert into Calls (caller_id, callee_id, duration)
values ('3', '12', '330');
insert into Calls (caller_id, callee_id, duration)
values ('12', '3', '5');
insert into Calls (caller_id, callee_id, duration)
values ('7', '9', '13');
insert into Calls (caller_id, callee_id, duration)
values ('7', '1', '3');
insert into Calls (caller_id, callee_id, duration)
values ('9', '7', '1');
insert into Calls (caller_id, callee_id, duration)
values ('1', '7', '7');
Solution
SELECT c.name AS country
FROM Calls,
Person,
Country c
WHERE (
caller_id = id
OR callee_id = id
)
AND country_code = LEFT(phone_number, 3)
GROUP BY country_code
HAVING AVG(duration) > (SELECT AVG(duration)
FROM Calls);