user: Question: Marcell Jansen with a Transfer window of Summer, and of Transfer as type is from what nation?
Columns: ['Nat.', 'Name', 'Type', 'Transfer window', 'Transfer fee']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['GER', 'Oliver Kahn', 'Career end', 'Summer', 'n/a']
SQL:

assistant: SELECT "Nat." FROM "Table" WHERE "Transfer window" = 'Summer' AND "Type" = 'Transfer' AND "Name" = 'Marcell Jansen';