Inner joins with Case statement - sql

Question

Instructions
Write a query to return the number of productive and less-productive
actors. The order of your results doesn't matter.
Definitions
productive: appeared in >= 30 films. less-productive: appeared in
<30 films.
I got this error on my query below
(syntax error at or near "AS" LINE 14: END) AS actor_category ^)
SELECT a.actor_id, MAX(a.first_name), MAX(a.last_name)
FROM actor a
INNER JOIN film_actor fa
ON a.actor_id = fa.actor_id
INNER JOIN film f
ON fa.film_id = f.film_id
(CASE
WHEN a.actor_id >= 30 THEN 'productive'
WHEN a.actor_id <= 30 THEN 'less-productive'
END) AS actor_category
GROUP BY a.actor_id;
This was the answer I was given:
SELECT actor_category,
COUNT(*)
FROM (
SELECT
A.actor_id,
CASE WHEN COUNT(DISTINCT FA.film_id) >= 30 THEN 'productive' ELSE 'less productive' END AS actor_category
FROM actor A
LEFT JOIN film_actor FA
ON FA.actor_id = A.actor_id
GROUP BY A.actor_id
) X
GROUP BY actor_category;
Why does it have to be done this way?

Answer 1

Plenty of SQL learning documents, but sometimes having a task and understanding that one can help you learn APPLIED techniques vs just here is how to write a query.
First, what do you care about. The total number of movies ANY given actor has been in. In this case, you have the film_actor table. Get your counts from this first.
select
fa.actor_id,
count(*) TotalMovies
from
film_actor fa
group by
fa.actor_id
The result of this might have
actor_id TotalMovies
1 7
2 34
3 27
4 41
In this case, you dont care WHO was in the movie, just that an actor was in X number of movies. So now that you have THIS query, you can then join to the actual actors table to pull their names and THEN apply the case/when based on the total movies count.
select
a.FirstName,
a.LastName,
PreQueryCnts.TotalMovies,
-- NOW we can apply the case/test
case when PreQueryCnts.TotalMovies >= 30
then 'productive'
else 'less productive' end as Actor_Category
from
(select
fa.actor_id,
count(*) TotalMovies
from
film_actor fa
group by
fa.actor_id ) PreQueryCnts
JOIN actor a
on PreQueryCnts.actor_id = a.actor_id
Although this does not specifically answer your question, you can see how you could get the detail. Now, to get your category total, just use the result of the original query and the simplify the count at the outer level too.
select
-- NOW we can apply the case/test
case when PreQueryCnts.TotalMovies >= 30
then 'productive'
else 'less productive' end as Actor_Category,
count(*) as ActorsInThisCategory
from
(select
fa.actor_id,
count(*) TotalMovies
from
film_actor fa
group by
fa.actor_id ) PreQueryCnts
group by
case when PreQueryCnts.TotalMovies >= 30
then 'productive'
else 'less productive' end
But there are many ways to pull a query. As long as you understand how/where the components come from. Notice the final query does not even care WHO the actor is and is not part of the query at all.