user: Question: What type of surface did they play on 21 July 1997
Columns: ['Outcome', 'Date', 'Championship', 'Surface', 'Opponent in the final', 'Score in the final']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Runner-up', '2 November 1992', 'Guarujá, Brazil', 'Hard', 'Carsten Arriens', '6–7, 3–6']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Date" = '21 July 1997';