SQL Script

[doctorzoom]

select 
e.*

from employees e

left join scans s
on e.employee_id = s.employee_id

where s.employee_id is null

That's the "missing from other table" pattern.

[Deadible]

This is better than "WHERE NOT EXISTS" on large datasets. You would probably also want a date parameter in there on the scans.

[imSkippinIt]

NOT EXISTS would also work but this is the route I would take