##Instruction:
Database: tracking_grants_for_research
Total Tables: 12
Total Columns: 42

Table Schemas:

tracking_grants_for_research contains tables such as Document_Types, Documents, Grants, Organisation_Types, Organisations, Project_Outcomes, Project_Staff, Projects, Research_Outcomes, Research_Staff, Staff_Roles, Tasks.
Table Document_Types has columns such as document_type_code, document_description.
document_type_code is the primary key.
Table Documents has columns such as document_id, document_type_code, grant_id, sent_date, response_received_date, other_details.
document_id is the primary key.
Table Grants has columns such as grant_id, organisation_id, grant_amount, grant_start_date, grant_end_date, other_details.
grant_id is the primary key.
Table Organisation_Types has columns such as organisation_type, organisation_type_description.
organisation_type is the primary key.
Table Organisations has columns such as organisation_id, organisation_type, organisation_details.
organisation_id is the primary key.
Table Project_Outcomes has columns such as project_id, outcome_code, outcome_details.
Table Project_Staff has columns such as staff_id, project_id, role_code, date_from, date_to, other_details.
staff_id is the primary key.
Table Projects has columns such as project_id, organisation_id, project_details.
project_id is the primary key.
Table Research_Outcomes has columns such as outcome_code, outcome_description.
outcome_code is the primary key.
Table Research_Staff has columns such as staff_id, employer_organisation_id, staff_details.
staff_id is the primary key.
Table Staff_Roles has columns such as role_code, role_description.
role_code is the primary key.
Table Tasks has columns such as task_id, project_id, task_details, eg Agree Objectives.
task_id is the primary key.
The grant_id of Documents is the foreign key of grant_id of Grants.
The document_type_code of Documents is the foreign key of document_type_code of Document_Types.
The organisation_id of Grants is the foreign key of organisation_id of Organisations.
The organisation_type of Organisations is the foreign key of organisation_type of Organisation_Types.
The outcome_code of Project_Outcomes is the foreign key of outcome_code of Research_Outcomes.
The project_id of Project_Outcomes is the foreign key of project_id of Projects.
The role_code of Project_Staff is the foreign key of role_code of Staff_Roles.
The project_id of Project_Staff is the foreign key of project_id of Projects.
The organisation_id of Projects is the foreign key of organisation_id of Organisations.
The employer_organisation_id of Research_Staff is the foreign key of organisation_id of Organisations.
The project_id of Tasks is the foreign key of project_id of Projects.

##Role Access Policy (Column-Level):
Role: ProjectManager
Accessible Columns: Projects: ALL COLUMNS; Project_Staff: ALL COLUMNS; Tasks: ALL COLUMNS; Project_Outcomes: ALL COLUMNS; Research_Outcomes: ALL COLUMNS; Staff_Roles: ALL COLUMNS; Research_Staff: staff_id, employer_organisation_id, staff_details; Organisations: organisation_id, organisation_type, organisation_details

Based on the above schema and your role's column-level access policy, generate a SQL query to answer the question. If your role does not have access to the required columns, respond with: "Sorry, I cannot answer."


Question: When did researchers start and stop working?
Answer: SELECT date_from ,  date_to FROM Project_Staff WHERE role_code  =  'researcher'
Permission: allowed (All required columns are permitted)
Gold SQL: SELECT date_from ,  date_to FROM Project_Staff WHERE role_code  =  'researcher'