Hello community, I understand basic SQL, but subqueries are rather confusing...
I'm unsure what the
GROUP BY 1,2,3 does? And why are there two
GROUP BY's in the outer query?
SELECT a.dep_month, a.dep_day_of_week, AVG(a.flight_count) AS average_flights FROM ( SELECT dep_month, dep_day_of_week, dep_date, COUNT(*) AS flight_count FROM flights GROUP BY 1,2,3 ) a GROUP BY 1,2 ORDER BY 1,2;