primary and foreign keys in postgresql - total beginner - postgresql

Question

can anyone explain me how to connect these tables?
I have to be able to list population in a country, list population in a city and list number of employees in a company for each city it is present in! The only limitation is that city name is unique and company can be present only once in one city but can have many branches in one country.
I have made country, company and city tables but I cannot understand how to connect them properly and get this functionality. I watched some youtube tutorials and read many posts but I still can't seem to make it work. If someone can help I would appreciate it. Thanks a lot guys!

Answer 1

One entity that is missing from your list is branch (of the company). The table definitions are then:
CREATE TABLE country (
id integer NOT NULL PRIMARY KEY,
name varchar NOT NULL,
population integer NOT NULL
);
CREATE TABLE city (
id integer NOT NULL PRIMARY KEY,
name varchar NOT NULL,
population integer NOT NULL,
country integer NOT NULL REFERENCES country(id)
);
Simple. You have a list of countries and a list of cities. The city records have a column country that references the proper record in the country table. City name is not set UNIQUE here because different countries can have a city by the same name (English, Dutch, Portuguese and Spanish empires created new cities with old names which were sometimes distinguishable (New York) but not always (such as Melbourne)). Append the UNIQUE clause to the name column if you really want the city name to be unique.
CREATE TABLE company (
id integer NOT NULL PRIMARY KEY,
name varchar NOT NULL
);
CREATE TABLE branch (
company integer NOT NULL REFERENCES company(id),
city integer NOT NULL REFERENCES city(id),
name varchar,
employees integer NOT NULL,
PRIMARY KEY (company, city)
);
The trick here is that the PK for the company branch is defined on the company identifier and the city identifier combined. A PK is always unique in the table.
Assuming that you want population counts and employees in a single query, this would do the trick:
SELECT
company.name AS company,
branch.name AS branch,
city.name AS city,
branch.employees,
city.population AS city_pop,
country.population AS country_pop
FROM company
JOIN branch ON branch.company = company.id
JOIN city ON city.id = branch.city
JOIN country ON country.id = city.country
ORDER BY company.name, branch.name;