When do all the researcher role staff start to work, and when do they stop working?
SELECT date_from ,  date_to FROM Project_Staff WHERE role_code  =  'researcher'
SELECT date_from ,  date_to FROM Project_Staff WHERE role_code  =  'researcher'
[{'date_from': '1981-10-09 21:32:53', 'date_to': '2004-12-16 13:03:36'}, {'date_from': '1983-02-07 17:55:59', 'date_to': '2004-07-28 03:11:47'}, {'date_from': '1991-01-11 16:57:50', 'date_to': '1993-06-09 12:44:28'}, {'date_from': '2005-01-13 11:49:48', 'date_to': '1973-07-19 04:51:26'}, {'date_from': '1970-03-25 06:18:11', 'date_to': '1985-12-05 12:00:58'}, {'date_from': '1972-01-17 19:42:16', 'date_to': '2016-03-15 00:33:18'}, {'date_from': '2000-08-28 11:49:17', 'date_to': '2007-02-02 17:26:02'}, {'date_from': '1989-04-24 23:51:54', 'date_to': '2002-03-19 18:00:36'}, {'date_from': '1973-12-12 11:46:28', 'date_to': '1971-07-19 22:49:05'}]