Question

我在我的大学项目中执行查询时遇到了一些问题。我有一个网站，用户可以分享他们阅读的书籍，我有一个页面，用户可以查看他添加的书籍（他拥有的书籍）。

为此我相信我需要记录用户的id，我存储在会话PHP变量中，用户id在表用户中。有关这些书籍的信息存储在一本书籍表中，它有自己的id主键。然后，显示谁拥有我拥有的表owns_book（id，u_id，book_id）。

现在进行测试我总共得到了26本书，其中25本是由身份为57的用户和1位用户身份为49的书籍添加的。当我运行此查询时：

SELECT id, title, author, category, cover, added, user_id FROM books, users 
WHERE user_id=49 AND id IN(SELECT book_id FROM owns_book) 
AND user_id IN(SELECT u_id FROM owns_book)

结果很糟糕，我没有得到预期的一本书，我也得到了其他用户添加的书籍。有人可以告诉我我需要的正确查询，或者我是否需要更改表格的结构？感谢。

编辑：

users(user_id, ...) 
books(id, title, author, publisher, published, cover... ) 
owns_book(id, u_id, book_id)

Answer 1

It looks like you're looking to to grab everything from your books table that is owned by a specific customer. If that's the case, you can try

SELECT * FROM books 
JOIN owns_book 
ON books.id = owns_books.book_id 
WHERE owns_book.user_id = 49

This will select all of the props from your books table then joins the tables based on on the ID of the book being equal to the book_id of the owns_book. Lastly, add the parameter - you only want to see user_id = 49.

Answer 2

您可以简化此查询并使用LEFT JOIN ...

SELECT books.id, title, author, category, cover, added, users.user_id 
    FROM users 
    LEFT JOIN owns_book on owns_book.user_id = users.user_id
    LEFT JOIN books on books.id = owns_book.id
    WHERE users.user_id=49

这将链接user_id并列出此user_id拥有的所有书籍（JOIN的ON位）。 WHERE子句仅限于列出您之后的user_id的记录。

如果在列的主列表中，多个表上有一列（如user_id），则在其前面加上表名，以允许数据库检测您要使用的列（即使它们可能是相同的值）。

Answer 3

You could also use inner join to join the tables users and books with the owns_book table:

SELECT id, title, author, category, cover, added, user_id 
    FROM owns_book 
    INNER JOIN users ON users.id = owns_book.u_id
    INNER JOIN books ON books.id = owns_book.book_id
    WHERE users.user_id=49

SQL - 返回用户拥有的书籍

3 个答案: