user: Question: What's the biggest zone?
Columns: ['Station', 'Year opened', 'Metropolitan borough [c ]', 'Zone', 'Servedby Route maps:- [ dead link ] [ dead link ]', 'Station users 2006–07', 'Station users 2007–08']
Types: ['text', 'text', 'text', 'real', 'text', 'real', 'real']
Sample row: ['Acocks Green', '1852', 'Birmingham', 3.0, 'Chiltern Railways London Midland', 160100.0, 161400.0]
SQL:

assistant: SELECT MAX("Zone") FROM "Table";