user: Question: What surface was played on when Piet Norval was a partner?
Columns: ['Outcome', 'Year', 'Championship', 'Surface', 'Partner', 'Opponents in the final', 'Score in the final']
Types: ['text', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Winner', 1999.0, 'French Open', 'Clay', 'Piet Norval', 'Larisa Neiland Rick Leach', '6–3, 3–6, 6–3']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Partner" = 'Piet Norval';