Find the percentage of restaurant in Napa Valley.
Napa Valley refers to region = 'Napa Valley'; percentage = divide(count(id_restaurant where region = 'Napa Valley'), count(id_restaurant)) * 100%
SELECT CAST(SUM(IIF(region = 'Napa Valley', 1, 0)) AS REAL) * 100 / COUNT(region) FROM geographic
database name:db_id restaurant
geographic table geographic Cols: city dtype text, country dtype text, region dtype text, 
generalinfo table generalinfo Cols: id restaurant dtype integer, label dtype text, food type dtype text, city dtype text, review dtype real, 
location table location Cols: id restaurant dtype integer, street number dtype integer, street name dtype text, city dtype text, 
