SQL-从两个表查询

Question

For a project, I had to write two tables and do a query on them. I have written the tables but need some guidance with the query part.

Here is the first table.

CREATE TABLE Country
(Name VARCHAR(35) NOT NULL UNIQUE,
Code VARCHAR(4) CONSTRAINT CountryKey PRIMARY KEY,
Capital VARCHAR(35),
Province VARCHAR(35),
Area NUMERIC CONSTRAINT CountryArea
    CHECK (Area >= 0),
Population NUMERIC CONSTRAINT CountryPop
    CHECK (Population >= 0));

Here is second table.

CREATE TABLE City
(Name VARCHAR(35),
Country VARCHAR(4),
Province VARCHAR(35),
Population NUMERIC CONSTRAINT CityPop
    CHECK (Population >= 0),
Longitude NUMERIC CONSTRAINT CityLon
    CHECK ((Longitude >= -180) AND (Longitude <= 180)) ,
Latitude NUMERIC CONSTRAINT CityLat
    CHECK ((Latitude >= -90) AND (Latitude <= 90)) ,
CONSTRAINT CityKey PRIMARY KEY (Name, Country, Province));

My query must do the following:

Return the min, max, and average latitude of all cities for all the countries in the table. 
And should be ordered by continent first and country second.

This is in PostgreSQL.

Thank you for your time.

Answer 1

You don't disclose the RDBMS in your question: is it MySQL or PostgreSQL?

However, this should work in both:

SELECT 
      continent -- not quite sure what this refers to
    , ctr.Code AS country_code
    , avg(city.latitude) AS avg_population
    , min(city.latitude) AS min_population
    , max(city.latitude) AS max_population
FROM
    country ctr
    JOIN city ON ctr.Code = city.Country
GROUP BY 
      continent
    , ctr.Code
ORDER BY 
      continent
    , ctr.Code
;

The GROUP BY clause makes the computed values refer to the ctr.Code.