Formatted question description: https://leetcode.ca/all/1294.html

1294. Weather Type in Each Country

Level

Easy

Description

Table: Countries

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| country_id    | int     |
| country_name  | varchar |
+---------------+---------+
country_id is the primary key for this table.
Each row of this table contains the ID and the name of one country.

Table: Weather

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| country_id    | int     |
| weather_state | varchar |
| day           | date    |
+---------------+---------+
(country_id, day) is the primary key for this table.
Each row of this table indicates the weather state in a country for one day.

Write an SQL query to find the type of weather in each country for November 2019.

The type of weather is Cold if the average weather_state is less than or equal 15, Hot if the average weather_state is greater than or equal 25 and Warm otherwise.

Return result table in any order.

The query result format is in the following example:

Countries table:
+------------+--------------+
| country_id | country_name |
+------------+--------------+
| 2          | USA          |
| 3          | Australia    |
| 7          | Peru         |
| 5          | China        |
| 8          | Morocco      |
| 9          | Spain        |
+------------+--------------+
Weather table:
+------------+---------------+------------+
| country_id | weather_state | day        |
+------------+---------------+------------+
| 2          | 15            | 2019-11-01 |
| 2          | 12            | 2019-10-28 |
| 2          | 12            | 2019-10-27 |
| 3          | -2            | 2019-11-10 |
| 3          | 0             | 2019-11-11 |
| 3          | 3             | 2019-11-12 |
| 5          | 16            | 2019-11-07 |
| 5          | 18            | 2019-11-09 |
| 5          | 21            | 2019-11-23 |
| 7          | 25            | 2019-11-28 |
| 7          | 22            | 2019-12-01 |
| 7          | 20            | 2019-12-02 |
| 8          | 25            | 2019-11-05 |
| 8          | 27            | 2019-11-15 |
| 8          | 31            | 2019-11-25 |
| 9          | 7             | 2019-10-23 |
| 9          | 3             | 2019-12-23 |
+------------+---------------+------------+
Result table:
+--------------+--------------+
| country_name | weather_type |
+--------------+--------------+
| USA          | Cold         |
| Austraila    | Cold         |
| Peru         | Hot          |
| China        | Warm         |
| Morocco      | Hot          |
+--------------+--------------+
Average weather_state in USA in November is (15) / 1 = 15 so weather type is Cold.
Average weather_state in Austraila in November is (-2 + 0 + 3) / 3 = 0.333 so weather type is Cold.
Average weather_state in Peru in November is (25) / 1 = 25 so weather type is Hot.
Average weather_state in China in November is (16 + 18 + 21) / 3 = 18.333 so weather type is Warm.
Average weather_state in Morocco in November is (25 + 27 + 31) / 3 = 27.667 so weather type is Hot.
We know nothing about average weather_state in Spain in November so we don't include it in the result table.

Solution

From table Weather, select each country’s average temperature of November 2019. Then join the result with table Countries using the field country_id. In the final result, replace the average temperature with the type of weather accordingly.

# Write your MySQL query statement below
select c.country_name, (
    case when w.avg_temperature <= 15 then 'Cold'
         when w.avg_temperature >= 25 then 'Hot'
         else 'Warm' end
) as weather_type
    from (
        select country_id, avg(weather_state) as avg_temperature from Weather
            where day between '2019-11-01' and '2019-11-30'
        group by country_id
    ) as w
        inner join Countries c
        on w.country_id = c.country_id;

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