Replacing Inner Join with a SubQuery in SQL

Question

Pardon if this question is too simple as I am in the process of learning how to write a subquery. I came across this guide online, which shows an example of how a SQL statement with INNER JOIN can be replaced by one with a Subquery in the SELECT clause.

There are 2 tables - Countries (code, name, continent, region, surface_area) and Cities (name, country_code, city_proper_pop, metroarea_pop).

SQL statement with INNER JOIN

SELECT countries.name AS country, COUNT(*) AS cities_num
      FROM cities
        INNER JOIN countries
        ON countries.code = cities.country_code
    GROUP BY country
    ORDER BY cities_num DESC, country
    LIMIT 9;

This returns the table:

Next, this statement is replaced by a SQL statement with a Subquery in the SELECT clause

    SELECT countries.name AS country,
      (SELECT COUNT(*)
       FROM cities
       WHERE countries.code = cities.country_code) AS cities_num
    FROM countries
    ORDER BY cities_num DESC, country
    LIMIT 9;

which shows the same resulting table .

My question is: I am unsure why the nested SQL query can contain this condition countries.code = cities.country_code in the WHERE clause without performing any Join. Because the nested query only draws result from cities table , it does not specify the countries table .

Hope someone can explain to me. Thank you in advance.

Answer 1

This is called correlated sub query.

For every row of outer query, it will run the inner query, in programming terms something like loop inside a loop.

More info..

https://www.geeksforgeeks.org/sql-correlated-subqueries/