Show me all the role codes.
select role_code from project_staff;
How about those of staff who starts working after 2003-04-19 15:06:20?
select role_code from project_staff where date_from > '2003-04-19 15:06:20';
List those of the staff who starts working after 2003-04-19 15:06:20 and ends working before 2016-03-15 00:33:18?
select role_code from project_staff where date_from > '2003-04-19 15:06:20' and date_to < '2016-03-15 00:33:18'