SQL 选择 n 到 m 关系

我有一个n-to-m之间的关系Author and Book.

表作者

ID       Name
1        Follett  
2        Rowling
3        Martin

桌书

ID     Title                       Category 
1        A Dance with Dragons      Fantasy
2        Harry Potter              Fantasy
3        The Key to Rebecca        Thriller
4        World without end         Drama

表 book_author

authorId       bookId
1        3  
2        2
3        1
1        4

系统中有更多的作者和书籍。现在我想选择所有拥有该类型书籍的作者“Fantasy".

这是我到目前为止想到的：

   select distinct a.id 
   from author a, book b, written w 
   where w.authorId = a.id and w.bookId = b.id and b.category = "Fantasy";

我想知道如何优化这个查询，因为特别是表书非常大。

建议使用显式JOIN而不是您当前拥有的隐式（逗号分隔的表列表）连接，因为如果您需要引入左连接，它将提高灵活性。

SELECT
  DISTINCT a.id
FROM
  author a
  JOIN book_author ba ON a.id = ba.authorId
  JOIN books b ON b.id = ba.bookId
WHERE b.category = 'Fantasy'

If your book_author已定义FOREIGN KEY关系回到author and books表，索引将被强制执行。同样，各自的id这些表中的列应定义为PRIMARY KEY。除此之外，您可以做的唯一潜在优化是创建索引books.category.

CREATE TABLE book_author (
  authorId INT NOT NULL, /* or whatever the data type... */
  bookId INT NOT NULL,
  /* define FK constraints in book_author */
  FOREIGN KEY (authorId) REFERENCES author (id),
  FOREIGN KEY (bookId) REFERENCES books (id)
);