user: Question: Name the surface for Philadelphia
Columns: ['Outcome', 'Year', 'Championship', 'Surface', 'Opponent in the final', 'Score in the final']
Types: ['text', 'real', 'text', 'text', 'text', 'text']
Sample row: ['Runner-up', 1973.0, 'Monte-Carlo', 'Clay', 'Ilie Năstase', '4–6, 1–6, 2–6']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Championship" = 'Philadelphia';