mysql - SQL query using sum() -

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hello i'm working on sql problem can't quite figure out. here schema i'm working with:

enter image description here

here question stuck on:

-- 3 find first name, last name , total combined film length of sci-fi films every actor. result should list names of of actors (even if actor has not been in sci-fi films) , total length of sci-fi films have been in.

so far have 

select actor.first_name, actor.last_name, (select sum(film.length) film  inner join film_category on film.film_id = film_category.film_id inner join category on film_category.category_id = category.category_id inner join film_actor on film_actor.film_id = film.film_id inner join actor on film_actor.actor_id = actor.actor_id category.name = 'sci-fi' ) actor

i know need group actor_id i'm unable in select subquery. have tips?

there no need use subquery. aggregate functions work on entire data set. 'group by' specifies how group data you're aggregating.

select a.actor_id, a.first_name, a.last_name, sum(f.length)   actor   left outer join film_actor fa on fa.actor_id   = a.actor_id   left outer join film       f  on f.film_id     = fa.film_id   left outer join film_categories       fc on fc.film_id    = f.film_id   left outer join categories            c  on c.category_id = fc.category_id  c.name = 'sci-fi'   group a.actor_id ;

the outer joins ensure actors no sci-fi film experience included in results


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