user: Question: How many different jockeys ran on 17 Feb 2007?
Columns: ['Result', 'Date', 'Race', 'Venue', 'Group', 'Distance', 'Weight (kg)', 'Jockey', 'Winner/2nd']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Won', '26 Aug 2006', 'Ian McEwen Trophy', 'Moonee Valley', 'G3', '1000 m', '55', 'C. Newitt', '2nd - Sassbee']
SQL:

assistant: SELECT COUNT("Jockey") FROM "Table" WHERE "Date" = '17 Feb 2007';