SELECT NULL = 5;
SELECT NULL = NULL;
SELECT employee_id, first_name, last_name, phone_number FROM employees WHERE phone_number = NULL;
SELECT employee_id, first_name, last_name, phone_number FROM employees WHERE phone_number IS NULL;
SELECT employee_id, first_name, last_name, phone_number FROM employees WHERE phone_number IS NOT NULL;
SELECT * FROM countries;
SELECT * FROM departments;
SELECT * FROM dependents;
SELECT * FROM employees;
SELECT * FROM jobs;
SELECT * FROM locations;
SELECT * FROM regions;
