MYSQL: Very Easy Solution 100% working
select employee_id from Employees where employee_id not in (select employee_id from Salaries)\nunion\nselect employee_id from Salaries where employee_id not in 
select employee_id from Employees where employee_id not in (select employee_id from Salaries)\nunion\nselect employee_id from Salaries where employee_id not in (select employee_id from Employees)\norder by employee_id;
['Union Find']