For each employee, list the number of documents which showed up in the circulation history.
select employees.employee_name, count(circulation_history.document_id) from employees join circulation_history on circulation_history.employee_id = employees.employee_id group by employees.employee_id;
Who showed up the most times?
select employees.employee_name from employees join circulation_history on circulation_history.employee_id = employees.employee_id group by employees.employee_id order by count(circulation_history.document_id) desc limit 1;
What about the names of the employees who have not showed up in any circulation history of documents.
select employee_name from employees except select employees.employee_name from employees join circulation_history on circulation_history.employee_id = employees.employee_id