SQL：选择子查询的总和

Question

我有一些相关的表格，其中包含电影，演员和类别数据。 我试图总结每个演员在特定类别中的电影长度。

这是我必须尝试在结果中包括类别ID子查询总和的内容：

SELECT actor.actor_id, (SELECT SUM(length) from film WHERE category_id=14) total
FROM actor
JOIN film_actor USING(actor_id)
JOIN film USING(film_id)
JOIN film_category USING(film_id)
JOIN category USING(category_id)
GROUP BY actor.actor_id
ORDER BY total DESC

但是，我的total列仅包含所有NULL值。

此查询有效，但不包括该类别中有0分钟价值电影的演员：

SELECT actor.actor_id, SUM(film.length) as total
FROM actor
JOIN film_actor USING(actor_id)
JOIN film USING(film_id)
JOIN film_category USING(film_id)
JOIN category USING(category_id)
WHERE category_id = 14
GROUP BY actor.actor_id

Answer 1

您需要一个外部联接，并且类别上的条件属于外部联接的on子句：

select a.actor_id,
       sum(f.length) as total
  from actor a
  join film_actor fa
    on a.actor_id = fa.actor_id
  join film f
    on fa.film_id = f.film_id
  left join film_category fc
    on fc.film_id = f.film_id
   and fc.category_id = 14
 group by a.actor_id

Answer 2

在第一个查询中，您将累加所有14类电影的长度，与演员无关，因此您将为每个演员看到相同的值。 如果所有值均为空，则类别14中没有电影。

我怀疑您想为每个演员加起来14类电影的长度....

SELECT actor.actor_id, SUM(IFNULL(film.length, 0))
FROM actor 
JOIN film_actor USING(actor_id)
JOIN film USING(film_id)
LEFT JOIN film_category 
   ON film_category.film_id=film.film_id
   AND film_category.category_id=14
GROUP BY actor.actor_id

Answer 3

这是我所需要的方法：

SELECT actor.actor_id, SUM(film.length) as total
FROM actor
JOIN film_actor USING(actor_id)
LEFT JOIN film on film_actor.film_id = film.film_id AND film.film_id IN  (SELECT film_id FROM film_category WHERE film_category.category_id = 14)
GROUP BY actor.actor_id