Найти второй наименее представленный регион во всех командах

Question

Мне нужно найти второй наименее представленный регион во всех командах. Регион может дублироваться из страны в страну.

Таким образом, цель состоит в том, чтобы подсчитать инженерные регионы в командах и получить вторую наименьшую величину.

Я до сих пор пытался сделать что-то вроде this

select 
    countries.region, 
    teams.name, 
    count(contries.region) 
from teams 
join engineers on teams.id = engineers.team_id 
join countries on countries.id = engineers.country_id
where countries.region in (
    select 
        countries.region
    from (select 
            countries.region,
            rownumber() over () as rownum 
            from countries
         ) as x 
    where
        x.rownum = 2
); 

Ссылки между таблицами не очень хорошие, поэтому я понятия не имею, как я могу это сделать

sqlhunt_development=# \d engineers
                                     Table "public.engineers"
   Column   |            Type             |                       Modifiers
------------+-----------------------------+--------------------------------------------------------
 id         | bigint                      | not null default nextval('engineers_id_seq'::regclass)
 first_name | character varying           |
 last_name  | character varying           |
 age        | integer                     |
 email      | character varying           |
 country_id | bigint                      |
 team_id    | bigint                      |
 created_at | timestamp without time zone | not null
 updated_at | timestamp without time zone | not null
Indexes:
    "engineers_pkey" PRIMARY KEY, btree (id)
    "index_engineers_on_country_id" btree (country_id)
    "index_engineers_on_team_id" btree (team_id)
Foreign-key constraints:
    "fk_rails_48c685314b" FOREIGN KEY (country_id) REFERENCES countries(id)
    "fk_rails_5c44cd68ac" FOREIGN KEY (team_id) REFERENCES teams(id)
Referenced by:
    TABLE "bookshelves" CONSTRAINT "fk_rails_13dec3ee94" FOREIGN KEY (engineer_id) REFERENCES engineers(id)
    TABLE "engineer_programming_languages" CONSTRAINT "fk_rails_3a4377ed71" FOREIGN KEY (engineer_id) REFERENCES engineers(id)

sqlhunt_development=# \d teams
                                        Table "public.teams"
      Column      |            Type             |                     Modifiers
------------------+-----------------------------+----------------------------------------------------
 id               | bigint                      | not null default nextval('teams_id_seq'::regclass)
 name             | character varying           |
 floor            | integer                     |
 features_shipped | integer                     |
 current_bugs     | integer                     |
 created_at       | timestamp without time zone | not null
 updated_at       | timestamp without time zone | not null
Indexes:
    "teams_pkey" PRIMARY KEY, btree (id)
Referenced by:
    TABLE "engineers" CONSTRAINT "fk_rails_5c44cd68ac" FOREIGN KEY (team_id) REFERENCES teams(id)

sqlhunt_development=# \d countries
                                     Table "public.countries"
   Column   |            Type             |                       Modifiers
------------+-----------------------------+--------------------------------------------------------
 id         | bigint                      | not null default nextval('countries_id_seq'::regclass)
 name       | character varying           |
 capital    | character varying           |
 region     | character varying           |
 population | integer                     |
 area       | integer                     |
 created_at | timestamp without time zone | not null
 updated_at | timestamp without time zone | not null
Indexes:
    "countries_pkey" PRIMARY KEY, btree (id)
Referenced by:
    TABLE "engineers" CONSTRAINT "fk_rails_48c685314b" FOREIGN KEY (country_id) REFERENCES countries(id)

Вывод вообще должен быть:

count |    region    
-------+------------+
   146 | Africa   
   159 | Oceania         
   159 | Europe         
   160 | NA       
   170 | SA       
   171 | SEA     
(8 rows)

Цель состоит в том, чтобы получить это:

count |    region    
-------+------------+  
   159 | Oceania          
(8 rows)

Answer 1

Вы можете использовать следующий запрос

select region,cnt from (
    select countries.region,
           count(countries.region) as cnt,
           dense_rank() over (order by count(countries.region)) AS ordr
    from teams join engineers on teams.id = engineers.team_id 
               join countries on countries.id = engineers.country_id
    group by countries.region 
        )a
where ordr=2    

Проверить демонстрацию здесь

Answer 2

Это можно сделать без оконных функций, используя что-то вроде этого:

WITH cte AS (
    SELECT
        countries.region, 
        COUNT(countries.region) AS ct
    FROM 
        teams 
        INNER JOIN engineers 
            ON teams.id = engineers.team_id 
        INNER JOIN countries 
            ON countries.id = engineers.country_id 
    GROUP BY
        countries.region
)
SELECT
    region, 
    ct
FROM
    cte
GROUP BY 
    region,
    ct
HAVING
    ct = (SELECT MIN(ct) FROM cte WHERE ct <> (SELECT MIN(ct) FROM cte))
;