SQL - 查找每个演员的科幻电影的名字，姓氏和总电影长度

Question

我有一个查询可以完成这个，但它排除了从未参加过科幻电影的演员。我想包括那些演员，即使科幻片的总长度为零。

结果应该列出所有演员的名字（即使演员没有参演任何科幻电影）以及他们所在科幻电影的总长度。

我有以下查询，我只是不知道如何修改它以包含所有演员。

SELECT actor.first_name, actor.last_name, SUM(film.length)
FROM actor
INNER JOIN film_actor ON actor.actor_id = film_actor.actor_id
INNER JOIN film on film_actor.film_id = film.film_id
INNER JOIN film_category ON film_actor.film_id = film_category.film_id
INNER JOIN category ON film_category.category_id = category.category_id
WHERE category.name = 'Sci-Fi'
GROUP BY category.name, actor.actor_id
ORDER BY actor.first_name

我正在使用以下架构：

Answer 1

除了切换到外部联接之外，您还可以使用条件聚合（可以轻松扩展到包括其他类别）：

SELECT actor.first_name, actor.last_name,
   SUM(case when category.name = 'Sci-Fi' then film.length else 0 end)
FROM actor
INNER JOIN film_actor ON actor.actor_id = film_actor.actor_id
INNER JOIN film on film_actor.film_id = film.film_id
INNER JOIN film_category ON film_actor.film_id = film_category.film_id
INNER JOIN category ON film_category.category_id = category.category_id
GROUP BY category.name, actor.actor_id
ORDER BY actor.first_name

这将排除那些从未制作过电影的演员。

Answer 2

您应该更改连接顺序并使用右连接与actor表而不是内连接：

SELECT actor.first_name, actor.last_name, SUM(film.length)
FROM category
INNER JOIN film_category ON film_category.category_id = category.category_id and category.name = 'Sci-Fi'
INNER JOIN film on film.film_id = film_category.film_id
INNER JOIN film_actor ON film.film_id = film_actor.film_id
RIGHT JOIN actor ON actor.actor_id = film_actor.actor_id
GROUP BY actor.first_name, actor.last_name
ORDER BY actor.first_name