What is the annual average number of races held during the first 10 years of the 21st century?
races in date between '2000-01-01' and '2010-12-31'
SELECT CAST(SUM(CASE WHEN year BETWEEN 2000 AND 2010 THEN 1 ELSE 0 END) AS REAL) / 10 FROM races WHERE date BETWEEN '2000-01-01' AND '2010-12-31'