如何根据条件删除结果以计算平均值

Question

我有下面的架构。一个简单的解释是：

1. 鲍勃评分提高了 5/5
2. 詹姆斯被评为 1/5
3. 梅西百货评分 5/5

逻辑：

1. 如果我是 personA，请查找我已阻止的所有人。
2. 查看所有电影评论。
3. 任何留下了影评且 personA 已屏蔽的人，将其从计算中移除。
4. 计算电影的平均评分。

---

CREATE TABLE movies (
  id integer AUTO_INCREMENT primary key,
  name varchar(100) NOT NULL
);

CREATE TABLE customer (
  id integer AUTO_INCREMENT primary key,
  name varchar(100) NOT NULL
);

CREATE TABLE reviews (
  id integer AUTO_INCREMENT primary key,
  rating integer NOT NULL,
  cus_id integer NOT NULL,
  movie_id integer NOT NULL,
  FOREIGN KEY (cus_id) REFERENCES customer(id),
  FOREIGN KEY (movie_id) REFERENCES movies(id)
);

CREATE TABLE blocked(
  id integer AUTO_INCREMENT primary key,
  cus_id integer NOT NULL, -- This is the person blocking
  blocked_cus_id integer NOT NULL, -- This is the person who is blocked
  FOREIGN KEY (cus_id) REFERENCES customer(id),
  FOREIGN KEY (blocked_cus_id) REFERENCES customer(id)
);

INSERT INTO movies (id, name) VALUES (1, 'up');
INSERT INTO customer (id, name) VALUES (1, 'bob'), (2, 'james'), (3, 'macy');
INSERT INTO reviews (id, rating, cus_id, movie_id) VALUES (1, 5, 1, 1), (2, 1, 2, 1), (3, 5, 3, 1);
INSERT INTO blocked (id, cus_id, blocked_cus_id) VALUES (1, 1, 2);

如果评论在黑名单上，我不明白如何摆脱评论

SELECT m.name, avg(r.rating) as avg_rating
FROM movies m
JOIN reviews r ON m.id = r.movie_id
JOIN blocked b on r.cus_id = b.blocked_cus_id
WHERE b.cus_id = 1 -- Viewing the database as bob

当我以 bob 形式查看数据库时，我应该得到：

+-------+------------+
| movie | avg_rating |
+-------+------------+
| up    |          5 |
+-------+------------+

当我将数据库视为 macy 时，我应该得到：

+-------+------------+
| movie | avg_rating |
+-------+------------+
| up    |       3.67 |
+-------+------------+

Answer 1

如果不使用存在子句，我不知道如何表达您的逻辑。我可以在这里试试：

SELECT m.name, AVG(r.rating) AS avg_rating
FROM movies m
INNER JOIN reviews r ON m.id = r.movie_id
WHERE NOT EXISTS (SELECT 1 FROM blocked b
                  WHERE b.blocked_cus_id = r.cus_id AND b.cus_id = 1)
GROUP BY m.name;

注意我们要按电影名称聚合，所以这里需要GROUP BY。 exists 子句表示从平均评分中排除我们可以找到属于某个客户（在本例中为cus_id = 1）的被阻止条目的任何评论。