For each document, list the number of employees who have showed up in the circulation history of that document. List the document ids and number of employees.
SELECT document_id ,  count(DISTINCT employee_id) FROM Circulation_History GROUP BY document_id;
number of employees who have showed up in the circulation history refers to COUNT(DISTINCT employee_id); listing document ids and number of employees requires SELECT document_id and aggregating employee counts with GROUP BY document_id;