How many Yelp businesses are opened 24 hours?
open 24 hours refers to attribute_name = 'Open 24 Hours' AND attribute_value = 'true';
SELECT COUNT(T2.business_id) FROM Attributes AS T1 INNER JOIN Business_Attributes AS T2 ON T1.attribute_id = T2.attribute_id WHERE T1.attribute_name LIKE 'Open 24 Hours' AND T2.attribute_value LIKE 'TRUE'