select
employees.firstname,
employees.lastname,
orders.orderdate,
orders.ordernum
from
employees
left join orders on orders.employees_id = employees.employee_id
where
orders.ordernum is null
group by 1,2,3,4
Explanation:
the left join gives you all the data in the employees table with matching records in the orders table
each order has has an employee ID because an employee would have made several orders
join the two tables with the ID numbers which are called keys
sql with search record where ordernum is empty in the two matching tables and return employee with no order record
I hope you understand this and it helps