user: Question: What Transfer window that has a Transfer fee of Free, with a Moving to of Derby County?
Columns: ['Nat.', 'Name', 'Moving to', 'Type', 'Transfer window', 'Transfer fee']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['SCO', 'Martin Ure', "Queen's Park", 'End of contract', 'Summer', 'n/a']
SQL:

assistant: SELECT "Transfer window" FROM "Table" WHERE "Transfer fee" = 'Free' AND "Moving to" = 'Derby County';