Calculate the percentage of movie titles with a screen length of more than 120 minutes that have a category of horror movies.
screen length of more than 120 minutes refers to length > 120; category of horror refers to category.name = 'Horror'; percentage = divide(count(title where length > 120 and category.name = 'Horror'), count(title)) * 100%
SELECT CAST(SUM(IIF(T3.`name` = 'Horror', 1, 0)) * 100 / COUNT(T1.film_id) AS REAL) FROM film AS T1 INNER JOIN film_category AS T2 ON T1.film_id = T2.film_id INNER JOIN category AS T3 ON T3.category_id = T2.category_id WHERE T1.length > 120