PSQL查詢從2個不同的表獲取信息

Question

我想從不同的表編寫查詢，但有一個查詢，我有一個問題。我們正在使用PSQL來進行這些查詢。我試圖執行的查詢是：

對於每個在數據庫中至少有一百個城市的國家，請列出其包含的城市總數。按照城市數量的升序排列結果。

我知道我將不得不使用計數函數來計算城市的數量，但我很困惑如何列出城市的總數。這裏是我已經試過代碼：

SELECT country.name AS name, COUNT(city.name) AS num_cities 
FROM what.country 
WHERE (num_cities > 100) 
ORDER BY num_cities ASC 

這裏是我現在用的是兩個表：

   Table "what.country" 
    Column  |   Type   |    Modifiers    
-----------------+-----------------------+-------------------------------------- 
country_code | character(3)   | not null default ''::bpchar 
name   | character varying(52) | not null default ''::character varying 
continent  | continent    | not null 
region   | character varying(26) | not null default ''::character varying 
surface_area | real     | not null default 0::real 
indep_year  | smallint    | 
population  | integer    | not null default 0 


       Table "what.city" 
    Column |   Type   |      Modifiers      
--------------+-----------------------+----------------------------------------- 
id   | integer    | not null default nextval('city_id_seq'::regclass) 
name   | character varying(35) | not null default ''::character varying 
country_code | character(3)   | not null default ''::bpchar 
district  | character varying(20) | not null default ''::character varying 
population | integer    | not null default 0 

Answer 1

您需要使用GROUP BY和HAVING，你需要得到國家至少100個城市

SELECT country.name AS name 
    COUNT(city.name) AS 
    num_cities 
    FROM country 
    JOIN city 
    ON country.country_code = 
    city.country_code 
    GROUP BY country.nam 
    HAVING COUNT(city.name) >=100 
    ORDER by COUNT(city.name) ASC