FAQ: Subqueries - Non-Correlated Subqueries III


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

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This exercise can be found in the following Codecademy content:

SQL: Table Transformation

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Hi, I don’t understand the meaning of ‘a.’ in the query :blush:

why I have to add this letter to complete the exercize?

Non-Correlated Subqueries III

SELECT a.dep_month,
AVG(a.flight_distance) AS average_distance
SELECT dep_month,
sum(distance) AS flight_distance
FROM flights
GROUP BY 1,2,3
) a


Please excuse all the edits (I’m a SQL newb) and I could be wrong but I believe the inner query…

SELECT dep_month,
SUM(distance) AS flight_distance
FROM flights
GROUP BY 1,2,3

…is being set to a variable of “a”. In turn, the outer queries runs a query on the “a” variable or table.

The inner query is used to create an organized table sorted by month, day of the week, and then I believe for cosmetic reasons(?), month date. The ‘flight_distance’ is a sum of all miles flown across all flights for that day in the month.

The outer query queries this cleaned up and organized temporary table ‘a’ and groups the final output by departure month, then all of the Mondays, Tuesdays, Wednesday’s, etc. for that month and then the average flight distances for each of those sets of days of the week.

In the end, we have the average total distance flown by day of week in each month.

To understand what was happening, it was helpful to run the inner query in the example first and see the table results and then add the outer query around it and again, see the results.