CREATE TABLE aircraft (aid DECIMAL, name TEXT, distance DECIMAL)
CREATE TABLE certificate (eid DECIMAL, aid DECIMAL)
CREATE TABLE employee (eid DECIMAL, name TEXT, salary DECIMAL)
CREATE TABLE flight (flno DECIMAL, origin TEXT, destination TEXT, distance DECIMAL, departure_date TIME, arrival_date TIME, price DECIMAL, aid DECIMAL)

How many employees have certificate.
SELECT COUNT(DISTINCT eid) FROM certificate
SELECT COUNT(DISTINCT "eid") FROM "certificate"