From what date and to what date do the staff work on a project that has the most staff and has staff in a leader role?
SELECT date_from ,  date_to FROM Project_Staff WHERE project_id IN( SELECT project_id FROM Project_Staff GROUP BY project_id ORDER BY count(*) DESC LIMIT 1 ) UNION SELECT date_from ,  date_to FROM Project_Staff WHERE role_code  =  'leader'
SELECT date_from ,  date_to FROM Project_Staff WHERE project_id IN( SELECT project_id FROM Project_Staff GROUP BY project_id ORDER BY count(*) DESC LIMIT 1 ) UNION SELECT date_from ,  date_to FROM Project_Staff WHERE role_code  =  'leader'
[{'date_from': '1970-01-02 15:35:05', 'date_to': '1985-09-22 09:06:08'}, {'date_from': '1970-04-06 15:50:21', 'date_to': '1983-03-19 16:06:31'}, {'date_from': '1973-12-12 11:46:28', 'date_to': '1971-07-19 22:49:05'}, {'date_from': '1981-10-04 22:44:50', 'date_to': '1985-05-30 22:26:30'}, {'date_from': '1981-10-09 21:32:53', 'date_to': '2004-12-16 13:03:36'}, {'date_from': '1999-10-21 22:07:15', 'date_to': '2008-09-25 20:06:28'}, {'date_from': '2003-04-19 15:06:20', 'date_to': '2010-12-08 11:55:36'}, {'date_from': '2004-11-01 23:52:38', 'date_to': '1988-03-04 19:30:05'}]