SQL Server 2012 触发器

Question

我有一个关于 SQL 的小问题困扰了我一段时间，假设我有两个表（Customer 和 Loan）。但是，我想要一个基于Borrowertype 属性进行检查的触发器。我想在 AND 之后的第二个查询中，我需要检查Loans 中的userID 是否与Customer 中的相同，但一定是搞砸了，否则我完全认为这是错误的方式。

CREATE TABLE Customer 
(
     userID int identity primary key, 
     Name varchar(20),
     Borrowertype varchar(20)
);

CREATE TABLE Loan 
(
     Id int identity primary key, 
     userID int,
     FOREIGN KEY (userID) REFERENCES Customer(userID)
);

IF OBJECT_ID ('Customer.maximum_books_per_user','TR') IS NOT NULL
  DROP TRIGGER Customer.maximum_books_per_user;
GO

CREATE TRIGGER maximum_books_per_user ON Customer
AFTER INSERT
AS
IF (SELECT Borrowertype FROM Customer) = 'diffborrowertypehere' 
    AND (SELECT COUNT(*) FROM inserted AS i JOIN Customer AS c 
        ON ??? WHERE ???
        ) > 5
BEGIN
   ROLLBACK TRANSACTION
   RAISERROR('You have reached maximum allowed loans.', 16, 1)
END
GO

Answer 1

您的触发器需要位于Loan 表上，因为在那里插入的行可能会被拒绝。像这样的：

编辑：重写以同时处理多个客户的插入

CREATE TRIGGER maximum_books_per_user ON Loan
FOR INSERT
AS
-- Fail if there are any customers that will have more than the maximum number of loans
IF EXISTS (
    SELECT i.userID, COUNT(*)
    FROM inserted i
    JOIN Loan l
        ON i.userID = l.userID
    GROUP BY i.userID
    HAVING COUNT(*) >= 5
)
BEGIN
    ROLLBACK TRANSACTION
    RAISERROR('You have reached maximum allowed loans.', 16, 1)
END