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;
SELECT document_id ,  count(DISTINCT employee_id) FROM Circulation_History GROUP BY document_id;
[{'document_id': 1, 'count(DISTINCT employee_id)': 1}, {'document_id': 2, 'count(DISTINCT employee_id)': 1}, {'document_id': 10, 'count(DISTINCT employee_id)': 1}, {'document_id': 20, 'count(DISTINCT employee_id)': 1}]