CREATE TABLE employee (eid NUMBER, name VARCHAR2, salary NUMBER)
CREATE TABLE certificate (eid NUMBER, aid NUMBER)
CREATE TABLE aircraft (aid NUMBER, name VARCHAR2, distance NUMBER)
CREATE TABLE flight (flno NUMBER, origin VARCHAR2, destination VARCHAR2, distance NUMBER, departure_date DATE, arrival_date DATE, price NUMBER, aid NUMBER)

I prefer a pie chart to understand how many flights from each origin city.
SELECT origin, COUNT(origin) FROM flight GROUP BY origin
SELECT "origin", COUNT("origin") FROM "flight" GROUP BY "origin"