尝试输出演员角色时得到错误答案

Question

我想弄清楚如何获得Daniel Radcliffe 的职业历史。但是，我得到的结果与 Daniel Radcliffe 无关。

CREATE TABLE person (
  id integer primary key,
  name text not null
);

CREATE TABLE movie (
  id integer primary key,
  name text not null
);

CREATE TABLE casts (
  movie_id integer not null,
  person_id integer not null,
  played_as text not null,
  foreign key (movie_id) references movie(id),
  foreign key (person_id) references person(id)
);

CREATE TABLE crew (
  movie_id integer not null,
  person_id integer not null,
  job text not null,
  foreign key (movie_id) references movie(id),
  foreign key (person_id) references person(id)
);

CREATE TABLE famous_level (
  movie_id integer not null,
  person_id integer not null,
  level integer not null,
  foreign key (movie_id) references movie(id),
  foreign key (person_id) references person(id)
);

INSERT INTO person (id, name) VALUES (1, 'Daniel Radcliffe'), (2, 'Emma Watson'), (3, 'Robert Downey Jr.'), (4, 'Joss Whedon');
INSERT INTO movie (id, name) VALUES (1, 'Harry Potter movie'), (2, 'The Avengers');
INSERT INTO casts (movie_id, person_id, played_as) VALUES (1, 1, 'Harry Potter'), (1, 2, 'Hermione Granger'), (2, 3, 'Tony Stark');
INSERT INTO crew (movie_id, person_id, job) VALUES (1, 1, 'Writer'), (1, 4, 'Director'), (2, 2, 'Director'), (2, 1, 'Writer');
INSERT INTO famous_level (movie_id, person_id, level) VALUES (1, 1, 1), (1, 2, 3), (2, 3, 2), (1, 4, 2);

---

我知道Daniel Radcliffe的ID是1

SELECT
  m.name,
  ct.played_as,
  cr.job
FROM movie m
JOIN crew cr ON m.id = cr.movie_id
JOIN casts ct ON m.id = ct.movie_id
JOIN famous_level f ON m.id = f.movie_id
JOIN person p ON cr.person_id = p.id
WHERE p.id = 1
ORDER BY f.level ASC

我想得到与此类似的结果，其中played_as 或job 列上都有NULL 值

+--------------------+--------------+--------+
|        name        |  played_as   |  job   |
+--------------------+--------------+--------+
| Harry Potter movie | Harry Potter | NULL   |
| Harry Potter movie | NULL         | Writer |
| The Avengers       | NULL         | Writer |
+--------------------+--------------+--------+

我得到的结果：http://www.sqlfiddle.com/#!17/28dd0

Answer 1

因为您想为每部电影的每个played_as 或job 获取一行，您需要创建一个包含所有played_as 和job 值的表，您可以使用UNION询问。当played_as 有效时，该查询为job 返回NULL，反之亦然。然后可以将其JOINed 到movie、person 和famous_level 表以过滤person_id 并按level 排序：

SELECT m.name,
       r.played_as,
       r.job
FROM movie m
JOIN (
  SELECT movie_id, person_id, played_as, NULL as job
  FROM casts
  UNION ALL 
  SELECT movie_id, person_id, NULL, job
  FROM crew
) r ON r.movie_id = m.id
JOIN person p ON p.id = r.person_id
LEFT JOIN famous_level f ON f.movie_id = m.id AND f.person_id = p.id
WHERE p.id = 1
ORDER BY COALESCE(f.level, 99)

输出：

name                played_as       job
Harry Potter movie  (null)          Director
Harry Potter movie  (null)          Writer
Harry Potter movie  Harry Potter    (null)
The Avengers        (null)          Writer

Demo on SQLFiddle

请注意，您需要在 movie_id 和 person_id 上都使用 JOIN 到 famous_level 以避免在输出中获得额外的行。由于并非所有电影/人物组合都有famous_level，因此您需要在level 值上使用LEFT JOIN 和COALESCE，以便当它为NULL 时。