FAQ: Subqueries - Correlated Subqueries II


#1

This community-built FAQ covers the “Correlated Subqueries II” 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 II

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#2

I just don’t understand what this question was ? " Using the same pattern, write a query to view flights by origin, flight id, and sequence number. Alias the sequence number column as `flight_sequence_number .". I had to get the solution to see what the code was doing. I seriously did not follow this chapter so far. There should be better examples.

Thank you
Sivarama Krishnan (kvsrkrishnan)


#3

I had the same problem, and have been having similar problems with the next few lessons as well.


#4

SELECT origin, id,
(SELECT COUNT(*)
FROM flights f
WHERE f.id < flights.id
AND f.origin=flights.origin) + 1
AS flight_sequence_number
FROM flights;

Here we query based on origin and id rather than carrier and id as shown in the example.


#5

But what does that achieve? What are you trying to query here? The Correlated subqueries are used for row-by-row processing, so for every orgin and id in the outer query… what are you executing in the inner query?


#6

Can somebody explain the inner query and atleast 2 iterations please ?


#7

I do agree with you. no explanation why the code is as it is… (unless i’ve skipped a bit)
why the ‘AS f’
why the ‘a.’

i can copy and paste and replace well, but it doesn’t mean i understand what is happening…
Guess i’ll have to read around other forums to find out why…
Its a bit disappointing, not as well thought through (or perhaps properly tested) as the initial SQL course…