What are the first name and department name of all employees?
SELECT T1.first_name ,  T2.department_name FROM employees AS T1 JOIN departments AS T2 ON T1.department_id  =  T2.department_id
first name of all employees refers to T1.first_name; department name refers to T2.department_name; JOIN is used to combine rows from two or more tables based on a related column between them, in this case, department_id.