Question

所以我是MySQL新手，遇到了一些麻烦。我有一个名为book_genres的表，另一个名为books。

book_genres
+-------+---------+
|book_id|  genre  |
+-------+---------+
| 1     | Horror  |
| 1     | Comedy  |
| 2     | Romance |
| 2     | Comedy  |
+-------+---------+

books
+-------+---------+
|book_id|  title  |
+-------+---------+
| 1     | A Book  |
| 2     | B Book  |
| 3     | C Book  |
+-------+---------+

我使用以下命令来提取所有具有3种选定类型的book_ids：

SELECT DISTINCT a.book_id, b.genre AS genre1, c.genre AS genre2, d.genre AS genre3 
FROM book_genres a 
JOIN book_genres b ON a.book_id = b.book_id AND b.genre LIKE 'Romance' 
JOIN book_genres c ON a.book_id = c.book_id AND c.genre LIKE 'Action' 
JOIN book_genres d ON a.book_id = d.book_id AND d.genre LIKE 'Comedy' 
GROUP BY book_id

我想要做的是现在使用此搜索中找到的book_ids从books表中提取所有书名。我不确定是否有更简单的方法可以做到这一点，但这就是我能想到的。

感谢所有可以提供帮助的人！

Answer 1

我认为这可能是一种更好的方法：

select b.*
from books b join
     book_genres bg
     on b.bookid = bg.bookid
where bg.genre in ('Romance', 'Action', 'Comedy')
group by b.book_id

这将选择所有具有三种类型之一的书籍。您的查询不清楚您是否想要“或”或“和”之间的类型，所以我假设“或”。

该组使用MySQL的一个名为“隐藏列”的功能。在这种情况下，它就像一个独特的。

如果你需要所有三种类型的书籍，你可以这样做：

select b.*
from books b join
     (select book_id
      from book_genre
      group by book_id
      having max(case when genre = 'Romance' then 1 else 0 end) > 0 and
             max(case when genre = 'Action' then 1 else 0 end) > 0 and
             max(case when genre = 'Comedy' then 1 else 0 end) > 0
     ) allg
     on b.book_id = allg.book_id

Answer 2

您可以执行子查询，如下所示：

SELECT books.title

FROM books

WHERE books.book_id 
IN (
SELECT 
    a.book_id

FROM 
    book_genres a 
    JOIN book_genres b ON a.book_id = b.book_id AND b.genre LIKE 'Romance' 
    JOIN book_genres c ON a.book_id = c.book_id AND c.genre LIKE 'Action' 
    JOIN book_genres d ON a.book_id = d.book_id AND d.genre LIKE 'Comedy' 

GROUP BY book_id
)

Answer 3

嗯，你会因为快速而肮脏的答案而踢你自己

SELECT DISTINCT a.book_id, a.book_title,
b.genre AS genre1, c.genre AS genre2, d.genre AS genre3 
FROM book_genres a 
JOIN book_genres b ON a.book_id = b.book_id AND b.genre = 'Romance' 
JOIN book_genres c ON a.book_id = c.book_id AND c.genre = 'Action' 
JOIN book_genres d ON a.book_id = d.book_id AND d.genre = 'Comedy' 
GROUP BY book_id

因为你没有使用外卡而被忽略了

我认为还有其他一些方法可以做到这一点，但是没有一种方法可以像你的努力那样容易理解，而且我很不确定它们会表现得更好，所以除非表现是可怕的而且不能用book_genres的类型栏索引来解决，我个人坚持这一点。

需要帮助使用SELECT DISTINCT结果搜索MySQL数据库

3 个答案: