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FAQ: Subqueries - Non-Correlated Subqueries I


This community-built FAQ covers the “Non-Correlated Subqueries I” exercise from the lesson “Subqueries”.

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This exercise can be found in the following Codecademy content:

SQL: Table Transformation

FAQs on the exercise Non-Correlated Subqueries I

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Since these tables are not joined by an primary key, how does it identify commonalities?


no hints in the class?

i have the same question!

I have the same question too.

Because the subquery returns the airports codes which are the same as the codes in the flights origin columns, this is the commonality


If the nesting query is unrelated to main query, why do we have to use ‘code’ from the airports table to get the results …why can’t any other column be used ?


Same question here! Why doesn’t it work when you replace ‘code’(from airports) with id (from airports)
both are unique?

I too have the same question - it should work with ID and code!

1 Like

Why are these producing different things? I don’t understand the usefulness of nesting queries over joining, but clearly the answer I get when I join is different so maybe there is a reason we’re supposed to use a nested query in this exercise:

select count(*) from flights join airports on airports.code=flights.origin where airports.elevation<2000;

select count(*) from flights where origin in (select code from airports where elevation<2000);

first produces 448 using join
second produces 438 using nesting (which they say is correct)

just checked results in excel, in the 448 there are 10 duplicates, ie flights wih id

dunno why they get duplicated, since if you check on sql, their entries are unique, but somehow when you join the two tables these 10 ids get a double row.

would be great to know why


Still no answer from the above questions.
Why can’t we replace ‘code’ with another column, such as ‘id’? Why is the subquery within the WHERE statement?

*** ‘code’ and ‘origin’ do not seem to be primary and foreign keys within themselves