-- Database schema
| Episode : episode_id [ TEXT ] primary_key , series [ TEXT ] , season [ INTEGER ] , episode [ INTEGER ] , number_in_series [ INTEGER ] , title [ TEXT ] , summary [ TEXT ] , air_date [ DATE ] , episode_image [ TEXT ] , rating [ REAL ] , votes [ INTEGER ] | Keyword : episode_id [ TEXT ] Keyword.episode_id = Episode.episode_id , keyword [ TEXT ] | Person : person_id [ TEXT ] primary_key , name [ TEXT ] , birthdate [ DATE ] , birth_name [ TEXT ] , birth_place [ TEXT ] , birth_region [ TEXT ] , birth_country [ TEXT ] , height_meters [ REAL ] , nickname [ TEXT ] | Award : award_id [ INTEGER ] primary_key , organization [ TEXT ] , year [ INTEGER ] , award_category [ TEXT ] , award [ TEXT ] , series [ TEXT ] , episode_id [ TEXT ] Award.episode_id = Episode.episode_id , person_id [ TEXT ] Award.person_id = Person.person_id , role [ TEXT ] , result [ TEXT ] | Credit : episode_id [ TEXT ] Credit.episode_id = Episode.episode_id , person_id [ TEXT ] Credit.person_id = Person.person_id , category [ TEXT ] , role [ TEXT ] , credited [ TEXT ] | Vote : episode_id [ TEXT ] Vote.episode_id = Episode.episode_id , stars [ INTEGER ] , votes [ INTEGER ] , percent [ REAL ] |
-- -- For season 9, episode 17 of the show Law and Order, how many roles have been included in the credit?

SELECT COUNT(T2.role) FROM Episode AS T1 INNER JOIN Credit AS T2 ON T1.episode_id = T2.episode_id WHERE T1.series = 'Law and Order' AND T1.season = 9 AND T1.episode = 17 AND T2.credited = 'true';