user: Question: Which Sleeves have a Back of unknown?
Columns: ['Year', 'Supplier', 'Chest', 'Sleeves', 'Back']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['unknown', 'Gilbert', 'unknown', 'unknown', 'unknown']
SQL:

assistant: SELECT "Sleeves" FROM "Table" WHERE "Back" = 'unknown';