FAQ: Subqueries - Correlated Subqueries I


This community-built FAQ covers the “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 Correlated Subqueries I

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I don’t understand the importance of
WHERE carrier = f.carrier

Could someone explain please?


The average value is calculated based on “carrier”, it is already compared with “distance”


This was confusing for me too.

I think since SQL is accessing the same table “flights” and column “distance” based on carriers SQL needs to distinguish the difference between the two. One instance of carriers is holding all the distances as “f.carrier” while the the other is holding the average (AVG) distances as just “carriers” so we are now comparing the two here: WHERE carrier = f.carrier I think SQL needs to be able to distinguish the two in order to use the < or > operators in the above query and give us the appropriate id associated to those carriers who are above or below average.

This is basically what @smilexdrus has stated and what I think I understood from it. I’m just trying to be more explanatory about it.


Why is there the following?
f.origin = flights.origin

Aren’t they both referring to the same chart and data?


My understanding is that this will calculate the average distance for each carrier every time it comes up in the flight list.
i.e. the average for each carrier is calculated multiple times.

Is that true?
If so, is this a wasteful/slow way of doing it?
If so, how would you go about doing it otherwise? Would you create a table of carrier names and averages then look up the value in that? Would that actually speed things up?

Thanks in advance,