而不是插入触发器问题

Question

我觉得我应该能够做到

SET @InsertCount = COUNT (*) FROM INSERTED WHERE ISBN = @ISBN

但它不会让我。我尝试使用 SELECT 语句，因为我看到其他人在线使用它，但我不知道还能做什么。

IF OBJECT_ID ('BooksTrigger') IS NOT NULL
    DROP TRIGGER BooksTrigger
GO

-- create the trigger
CREATE TRIGGER BooksTrigger 
ON Books
INSTEAD OF INSERT
AS
    DECLARE @ISBN INT,
            @BookInDate DATE,
            @Memo VARCHAR(100),
            @Copies INT,
            @PreyQty INT,
            @InsertISBNCount INT,
            @CurrQty INT

    SET @InsertISBNCount = (SELECT COUNT(ISBN) FROM inserted WHERE ISBN = @ISBN)
    SET @PreyQty = (SELECT COUNT(ISBN) FROM Books WHERE ISBN = @ISBN)
    SET @CurrQty = (SELECT SUM(@InsertISBNCount + @PreyQty))
    SET @Copies = (SELECT CopiesNo FROM BookInfo WHERE ISBN = @ISBN)

    IF @Copies > @CurrQty
    BEGIN
        PRINT 'Inserting a copy of the book with ISBN ' + @ISBN

        INSERT INTO Books (ISBN, BookInDate, Memo)
        VALUES (@ISBN, @BookInDate, @Memo)
    END
    ELSE
        THROW 50027, 'Number of copies available for this book is less than the inserts for this book. Insert did not go through.', 1
GO

-- test
BEGIN TRY
    INSERT INTO Books 
    VALUES ('9780393248968', GETDATE(), 'test') --should print 'inserting'

    INSERT INTO Books 
    VALUES ('9780393248968', GETDATE(), 'test') --this one should throw an error as this would be the fifth book
END TRY
BEGIN CATCH
    PRINT ERROR_MESSAGE()
END CATCH

--see if the first one inserted
SELECT * FROM Books

所以这里的目标是创建一个 INSTEAD OF 触发器，用于在 Books 表中插入数据。我想查看插入到 Books 表中的 ISBN 的书籍数量是否超过 BookInfo 中相同 ISBN 的 CopiesNo，抛出错误，否则插入带有打印消息“正在插入...”或其他内容的数据。

我被告知完成后我应该看到这样的消息：

但我目前得到这个：

这是制作两个表（Books AND BookInfo）的代码

--BookInfo Table
CREATE TABLE BookInfo
(
    ISBN BIGINT PRIMARY KEY CHECK (LEN([ISBN]) BETWEEN 10 AND 13),
    BookCategoryID INT,
    BookName VARCHAR(60),           
    BookAuthor VARCHAR(40),
    BookPublisher VARCHAR(40),
    BookPubDate DATE,
    BookPages INT CHECK (BookPages > 0),
    BookPrice INT CHECK (BookPrice > 0),
    CopiesNo INT DEFAULT 1 CHECK (CopiesNo > 0),
    Memo VARCHAR(100) NULL,
);

--Books Table
CREATE TABLE Books
(
    BookID INT PRIMARY KEY IDENTITY(1,1),
    ISBN BIGINT,
    BookInDate DATE,
    Memo VARCHAR(100) NULL, 
);
GO

ALTER TABLE Books
ADD FOREIGN KEY (ISBN) REFERENCES BookInfo(ISBN);
GO

INSERT INTO BookInfo (ISBN, BookCategoryID, BookName, BookAuthor,BookPublisher, BookPubDate, BookPages, BookPrice, CopiesNo, Memo)
VALUES (9780393248968, 1, 'Storm in a Teacup: The Physics of Everyday Life', 'Helen Czerski', 'W. W. Norton & Company', '01/10/2017', 288, 26.95, 4, 'On sale'),
       (9780062328502, 1, 'Dark Matter and the Dinosaurs: The Astounding', 'Lisa Randall', 'Ecco', '10/18/2016', 432, 17.99, 2, 'Not on Sale'),
       (9780262517263, 2, 'Grace Hopper and the Invention of the Information Age', 'Kurt Beyer', 'MIT Press', '02/01/2012', 404, 27.95, 2, 'On sale');

INSERT INTO Books (ISBN, BookInDate, Memo)
VALUES (9780393248968, '01/01/2018', 'this is a book'),
       (9780393248968, '01/01/2018', 'this is a book'),
       (9780393248968, '01/01/2018', 'this is a book'),
       (9780062328502, '01/02/2018', 'this is a book I think'),
       (9780062328502, '01/02/2018', 'this is a book I think'),
       (9780262517263, '01/03/2018', 'this is a book for sure'),
       (9780262517263, '01/03/2018', 'this is a book for sure');

Answer 1

我假设您试图阻止添加的 Book 记录多于相关 ISBN 的 BookInfo 记录中配置的副本数。

但是，您的逻辑只是检查作为此插入语句的一部分插入的Book 的数量（inserted 表仅用于此插入）。因此，如果只允许一本书的 4 个副本，但您将它们作为 3 条记录插入，然后再插入 3 条记录（总共 6 条），您的触发器将允许这样做。

如果您按照自己的逻辑解决此问题，则需要计算 Book 表和 Inserted 表中的 Book。

但是，您可以通过使用after 触发器，对Book 表进行计数，并在条件失败时回滚，从而以更简单的方式解决问题，例如

create trigger BooksTrigger on Books
after Insert
as
begin
  -- if, for any BookInfo record, there exist more book records than CopiesNo, rollback
  if exists (select 1 from BookInfo BI where BI.CopiesNo < (select count(*) from Book B where B.ISBN = BI.ISBN)) begin
    ;THROW 50027, 'Number of copies available for this book is less than the inserts for this book. Insert did not go through.', 1
  end
end