文章目錄

• • 1. 題目
  • 2. 解題

1. 題目

表 Person:

+----------------+---------+ | Column Name | Type | +----------------+---------+ | id | int | | name | varchar | | phone_number | varchar | +----------------+---------+ id 是該表主鍵. 該表每一行包含一個(gè)人的名字和電話號(hào)碼. 電話號(hào)碼的格式是:'xxx-yyyyyyy', 其中xxx是國(guó)家碼(3個(gè)字符), yyyyyyy是電話號(hào)碼(7個(gè)字符), x和y都表示數(shù)字. 同時(shí), 國(guó)家碼和電話號(hào)碼都可以包含前導(dǎo)0.

表 Country:

+----------------+---------+ | Column Name | Type | +----------------+---------+ | name | varchar | | country_code | varchar | +----------------+---------+ country_code是該表主鍵. 該表每一行包含國(guó)家名和國(guó)家碼. country_code的格式是'xxx', x是數(shù)字.

表 Calls:

+-------------+------+ | Column Name | Type | +-------------+------+ | caller_id | int | | callee_id | int | | duration | int | +-------------+------+ 該表無主鍵, 可能包含重復(fù)行. 每一行包含呼叫方id, 被呼叫方id 和 以分鐘為單位的通話時(shí)長(zhǎng). caller_id != callee_id

一家電信公司想要投資新的國(guó)家.
該公司想要投資的國(guó)家是: 該國(guó)的平均通話時(shí)長(zhǎng)要嚴(yán)格地大于全球平均通話時(shí)長(zhǎng).

寫一段 SQL, 找到所有該公司可以投資的國(guó)家.

返回的結(jié)果表沒有順序要求.

查詢的結(jié)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 | +----------+ 國(guó)家Peru的平均通話時(shí)長(zhǎng)是 (102 + 102 + 330 + 330 + 5 + 5) / 6 = 145.666667 國(guó)家Israel的平均通話時(shí)長(zhǎng)是 (33 + 4 + 13 + 13 + 3 + 1 + 1 + 7) / 8 = 9.37500 國(guó)家Morocco的平均通話時(shí)長(zhǎng)是 (33 + 4 + 59 + 59 + 3 + 7) / 6 = 27.5000 全球平均通話時(shí)長(zhǎng) = (2 * (33 + 4 + 59 + 102 + 330 + 5 + 13 + 3 + 1 + 7)) / 20 = 55.70000 所以, Peru是唯一的平均通話時(shí)長(zhǎng)大于全球平均通話時(shí)長(zhǎng)的國(guó)家, 也是唯一的推薦投資的國(guó)家.

來源：力扣（LeetCode）
鏈接：https://leetcode-cn.com/problems/countries-you-can-safely-invest-in
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2. 解題

• 先找出打電話的人是哪個(gè)國(guó)家的

select id, c.name country from Person p left join Country c on left(p.phone_number,3) = c.country_code {"headers": ["id", "country"], "values": [ [3, "Peru"], [12, "Peru"], [1, "Morocco"], [2, "Morocco"], [7, "Israel"], [9, "Israel"]]}

• 計(jì)算打出去的人的分鐘數(shù)和人數(shù)

# Write your MySQL query statement below with people_country as (select id, c.name countryfrom Person p left join Country con left(p.phone_number,3) = c.country_code )select country, count(*) num, sum(duration) calltime from Calls c1 left join people_country on c1.caller_id = people_country.id group by country {"headers": ["country", "num", "calltime"], "values": [ ["Morocco", 4, 103], ["Peru", 3, 437], ["Israel", 3, 17]]}

• 統(tǒng)計(jì)接聽的人的人數(shù)和分鐘數(shù)，并合并

# Write your MySQL query statement below with people_country as (select id, c.name countryfrom Person p left join Country con left(p.phone_number,3) = c.country_code )select country, count(*) num, sum(duration) calltime from Calls c1 left join people_country on c1.caller_id = people_country.id group by country union all select country, count(*) num, sum(duration) calltime from Calls c2 left join people_country on c2.callee_id = people_country.id group by country {"headers": ["country", "num", "calltime"], "values": [ ["Morocco", 4, 103], ["Peru", 3, 437], ["Israel", 3, 17], ["Israel", 5, 58], ["Morocco", 2, 62], ["Peru", 3, 437]]}

• 最終答案

# Write your MySQL query statement below with people_country as (select id, c.name countryfrom Person p left join Country con left(p.phone_number,3) = c.country_code )select country from (select country, sum(num) totalnum, sum(calltime) totaltime, sum(calltime)/sum(num) avgtimefrom(select country, count(*) num, sum(duration) calltimefrom Calls c1 left join people_countryon c1.caller_id = people_country.idgroup by countryunion allselect country, count(*) num, sum(duration) calltimefrom Calls c2 left join people_countryon c2.callee_id = people_country.idgroup by country) tgroup by country ) temp where avgtime > (select avg(duration) avgtimefrom(select caller_id, durationfrom Callsunion allselect callee_id, durationfrom Calls) t)

• 更簡(jiǎn)潔一點(diǎn)

# Write your MySQL query statement below with people_country as (select id, c.name countryfrom Person p left join Country con left(p.phone_number,3) = c.country_code )select country from (select country, avg(duration) avgtimefrom(select caller_id id, durationfrom Callsunion allselect callee_id, durationfrom Calls) t left join people_countryusing(id)group by country ) temp where avgtime > (select avg(duration) avgtimefrom(select caller_id, durationfrom Callsunion allselect callee_id, durationfrom Calls) t)

• 評(píng)論區(qū)更簡(jiǎn)潔的答案

# Write your MySQL query statement below 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)

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