user: Question: What was the surface on 23 October 2011?
Columns: ['Outcome', 'Date', 'Surface', 'Opponent in the final', 'Score in the final']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Runner-up', '17 August 2008', 'Hard', 'Juan Martín del Potro', '3–6, 3–6']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Date" = '23 October 2011';