多个 SQL 连接

Question

我需要执行一个查询来从多个表中检索数据，但我对如何一次完成所有操作感到相当困惑。

Books: _ISBN , BookTitle, Edition, Year, PublisherID, Pages, Rating
Categories: _CategoryID, Category
Categories_Books: _Categories_Category_ID, _Books_ISBN
Publishers: _Publisherid, Publisher
Writers: _WriterID, LastName
Writers_Books: _Writers_WriterID, _Books_ISBN

Categories_Books 和 Writers_Books 是中间表，可帮助我实现表之间的多对多关系。

我需要一个具有多个连接的查询来选择：

• 标题、版本、年份、页数、
• 类别中的图书类别
• 、来自出版商的出版商、
• 来自作家的姓氏

Answer 1

它将是这样的：

SELECT b.Title, b.Edition, b.Year, b.Pages, b.Rating, c.Category, p.Publisher, w.LastName
FROM
    Books b
    JOIN Categories_Book cb ON cb._ISBN = b._Books_ISBN
    JOIN Category c ON c._CategoryID = cb._Categories_Category_ID
    JOIN Publishers p ON p._PublisherID = b.PublisherID
    JOIN Writers_Books wb ON wb._Books_ISBN = b._ISBN
    JOIN Writer w ON w._WritersID = wb._Writers_WriterID

您使用 join 语句来指示表 A 中的哪些字段映射到表 B。我在这里使用别名，这就是您看到 Books b 的原因在查询的其余部分中，Books 表将被称为 b。这可以减少打字次数。

仅供参考，您的命名约定很奇怪，我希望它更像这样：

Book: ID, ISBN , BookTitle, Edition, Year, PublisherID, Pages, Rating
Category: ID, [Name]
BookCategory: ID, CategoryID, BookID
Publisher: ID, [Name]
Writer: ID, LastName
BookWriter: ID, WriterID, BookID

Answer 2

你可以使用这样的东西：

SELECT
    Books.BookTitle,
    Books.Edition,
    Books.Year,
    Books.Pages,
    Books.Rating,
    Categories.Category,
    Publishers.Publisher,
    Writers.LastName
FROM Books
INNER JOIN Categories_Books ON Categories_Books._Books_ISBN = Books._ISBN
INNER JOIN Categories ON Categories._CategoryID = Categories_Books._Categories_Category_ID
INNER JOIN Publishers ON Publishers._Publisherid = Books.PublisherID
INNER JOIN Writers_Books ON Writers_Books._Books_ISBN = Books._ISBN
INNER JOIN Writers ON Writers.Writers_Books = _Writers_WriterID.

Answer 3

 SELECT
 B.Title, B.Edition, B.Year, B.Pages, B.Rating     --from Books
, C.Category                                        --from Categories
, P.Publisher                                       --from Publishers
, W.LastName                                        --from Writers

FROM Books B

JOIN Categories_Books CB ON B._ISBN = CB._Books_ISBN
JOIN Categories_Books CB ON CB.__Categories_Category_ID = C._CategoryID
JOIN Publishers P ON B.PublisherID = P._Publisherid
JOIN Writers_Books WB ON B._ISBN = WB._Books_ISBN
JOIN Writers W ON WB._Writers_WriterID = W._WriterID