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

What is the count of distinct employees with certificates?
SELECT COUNT(DISTINCT eid) FROM certificate
SELECT COUNT(DISTINCT "eid") FROM "certificate"