SQL Server 2012 Trigger

Question

I have a small little thing with SQL that's been bothering me now for a while, let's say I have two tables ( Customer and Loan ). However, I want a trigger that's checking based on the Borrowertype attribute. I suppose with the second query after AND I need something to check whether the userID in Loans are the same as the one in Customer , but must be messing it up or I'm completely thinking this the wrong way.

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

Your trigger needs to be on the Loan table, as that's where a row would be being inserted that could be rejected. Something like this:

EDIT : rewritten to handle inserts for multiple Customers at once

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