•What’s the use for “a.” in: a.dep_month, a.dep_day_of_week, a.flight_count?
It is called an Table-alias normaly they use the =keyword= as
COUNT(*) AS flight_count
GROUP BY 1,2,3
) AS a
you created the =virtual= table a
Aliasing a table will result a reference to the original table that exists for the duration of the SQL statement.
•What’s the use for GROUP BY 1,2,3? There’s no column named 1,2,3
maybe you get the idear by reading
and PLEASE notice that it is DANGEROUS to use, imagine the column-order in the table being changed in future developments!!