query: SELECT date_from ,  date_to FROM Project_Staff WHERE role_code  =  'researcher'
question: When do all researcher role staffs start to works, and so when are stopping working?
query_toks: SELECT date_from , date_to FROM Project_Staff WHERE role_code = 'researcher '
query_toks_no_value: select date_from , date_to from project_staff where role_code = value
question_toks: When do all researcher role staffs start to works, and so when are stopping working?