What is the average rating for the all Yelp businesses that open 24 hours?
open 24 hours refers to attribute_name = 'Open 24 Hours' AND attribute_value = 'true'; rating refers to stars; average rating = AVG(stars);
SELECT CAST(SUM(T3.stars) AS REAL) / COUNT(T2.business_id) AS "avg" FROM Attributes AS T1 INNER JOIN Business_Attributes AS T2 ON T1.attribute_id = T2.attribute_id INNER JOIN Business AS T3 ON T2.business_id = T3.business_id WHERE T1.attribute_name LIKE 'Open 24 Hours' AND T2.attribute_value LIKE 'TRUE'