system: You are a text-to-SQL generator. Given a database schema and a question, reply with a single valid SQLite query that answers it. Output only the SQL on one line, with no explanation and no markdown.

user: Schema:
PRAGMA foreign_keys = ON;
CREATE TABLE `Document_Types` (
`document_type_code` VARCHAR(10) PRIMARY KEY,
`document_description` VARCHAR(255) NOT NULL
);
CREATE TABLE `Documents` (
`document_id` INTEGER PRIMARY KEY,
`document_type_code` VARCHAR(10),
`grant_id` INTEGER NOT NULL,
`sent_date` DATETIME NOT NULL,
`response_received_date` DATETIME NOT NULL,
`other_details` VARCHAR(255) NOT NULL,
FOREIGN KEY (`document_type_code` ) REFERENCES `Document_Types`(`document_type_code` ),
FOREIGN KEY (`grant_id` ) REFERENCES `Grants`(`grant_id` )
);
CREATE TABLE `Grants` (
`grant_id` INTEGER PRIMARY KEY,
`organisation_id` INTEGER NOT NULL,
`grant_amount` DECIMAL(19,4) NOT NULL DEFAULT 0,
`grant_start_date` DATETIME NOT NULL,
`grant_end_date` DATETIME NOT NULL,
`other_details` VARCHAR(255) NOT NULL,
FOREIGN KEY (`organisation_id` ) REFERENCES `Organisations`(`organisation_id` )
);
CREATE TABLE `Organisation_Types` (
`organisation_type` VARCHAR(10) PRIMARY KEY,
`organisation_type_description` VARCHAR(255) NOT NULL
);
CREATE TABLE `Organisations` (
`organisation_id` INTEGER PRIMARY KEY,
`organisation_type` VARCHAR(10) NOT NULL,
`organisation_details` VARCHAR(255) NOT NULL,
FOREIGN KEY (`organisation_type` ) REFERENCES `Organisation_Types`(`organisation_type` )
);
CREATE TABLE `Project_Outcomes` (
`project_id` INTEGER NOT NULL,
`outcome_code` VARCHAR(10) NOT NULL,
`outcome_details` VARCHAR(255),
FOREIGN KEY (`project_id` ) REFERENCES `Projects`(`project_id` ),FOREIGN KEY (`outcome_code` ) REFERENCES `Research_Outcomes`(`outcome_code` )
);
CREATE TABLE `Project_Staff` (
`staff_id` DOUBLE PRIMARY KEY,
`project_id` INTEGER NOT NULL,
`role_code` VARCHAR(10) NOT NULL,
`date_from` DATETIME,
`date_to` DATETIME,
`other_details` VARCHAR(255),
FOREIGN KEY (`project_id` ) REFERENCES `Projects`(`project_id` ),FOREIGN KEY (`role_code` ) REFERENCES `Staff_Roles`(`role_code` )
);
CREATE TABLE `Projects` (
`project_id` INTEGER PRIMARY KEY,
`organisation_id` INTEGER NOT NULL,
`project_details` VARCHAR(255) NOT NULL,
FOREIGN KEY (`organisation_id` ) REFERENCES `Organisations`(`organisation_id` )
);
CREATE TABLE `Research_Outcomes` (
`outcome_code` VARCHAR(10) PRIMARY KEY,
`outcome_description` VARCHAR(255) NOT NULL
);
CREATE TABLE `Research_Staff` (
`staff_id` INTEGER PRIMARY KEY,
`employer_organisation_id` INTEGER NOT NULL,
`staff_details` VARCHAR(255) NOT NULL,
FOREIGN KEY (`employer_organisation_id` ) REFERENCES `Organisations`(`organisation_id` )
);
CREATE TABLE `Staff_Roles` (
`role_code` VARCHAR(10) PRIMARY KEY,
`role_description` VARCHAR(255) NOT NULL
);
CREATE TABLE `Tasks` (
`task_id` INTEGER PRIMARY KEY,
`project_id` INTEGER NOT NULL,
`task_details` VARCHAR(255) NOT NULL,
`eg Agree Objectives` VARCHAR(1),
FOREIGN KEY (`project_id` ) REFERENCES `Projects`(`project_id` )
);

Question: 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?

assistant: 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'