Insert trigger doesnt do what i want it to do - sql

Question

i made a trigger which should avoid inserting a record in the rental 'uitlening' table if the person has an overdue payment (Boete). Unfortunately it doesnt work and i cant find the reason why. 'Boete' is an attribute of another table than rental. Can someone help me?
CREATE TRIGGER [dbo].[Trigger_uitlening]
ON [dbo].[Uitlening]
FOR INSERT
AS
BEGIN
DECLARE #Boete decimal(10, 2);
SET #Boete = (SELECT Boete FROM Lid WHERE LidNr = (SELECT LidNr FROM inserted));
IF #Boete = 0
BEGIN
INSERT INTO Uitlening
SELECT *
FROM inserted;
END;
END;

Answer 1

It sounds like what you actually need is a cross-table constraint.
You can either do this by throwing an error in the trigger:
CREATE TRIGGER [dbo].[Trigger_uitlening]
ON [Rental]
AFTER INSERT
AS
SET NOCOUNT ON;
IF EXISTS (SELECT 1
FROM inserted i
INNER JOIN dbo.Person p ON i.[personID] = p.[personID]
WHERE p.[PaymentDue] <= 0
)
THROW 50001, 'PaymentDue is less than 0', 1;
A better solution is to utilize a trick with an indexed view. This is based on an article by spaghettidba.
We first create a dummy table of two rows
CREATE TABLE dbo.DummyTwoRows (dummy bit not null);
INSERT DummyTwoRows (dummy) VALUES(0),(1);
Then we create the following view:
CREATE VIEW dbo.vwPaymentLessThanZero
WITH SCHEMBINDING -- needs schema-binding
AS
SELECT 1 AS DummyOne
FROM dbo.Rental r
JOIN dbo.Person p ON p.personID = r.personID
CROSS JOIN dbo.DummyTwoRows dtr
WHERE p.PaymentDue <= 0;
This view should in theory always have no rows in it. To enforce that, we create an index on it:
CREATE UNIQUE CLUSTERED INDEX CX_vwPaymentLessThanZero
ON dbo.vwPaymentLessThanZero (DummyOne);
Now if you try to add a row that qualifies for the view, it will fail with a unique violation, because the cross-join is doubling up the rows.
Note that in practice the view index takes up no space because there are never any rows in it.

Answer 2

Assuming you just want to insert records into [Rental] of those users, who have [PaymentDue] <= 0. As you mentioned in your last comment:
no record in rental can be inserted if the person has a PaymentDue
thats greater than zero
And other records should be silently discarded as you didn't give a clear answer to #Larnu's question:
should that row be silently discarded, or should an error be thrown?
If above assumptions are true, your trigger would look like:
CREATE TRIGGER [dbo].[Trigger_uitlening]
ON [Rental]
INSTEAD OF INSERT
AS
BEGIN
INSERT INTO [Rental] ( [DATE], [personID], [productID])
SELECT i.[DATE], i.[personID], i.[productID]
FROM INSERTED i
INNER JOIN Person p ON i.[personID] = p.[personID]
WHERE p.[PaymentDue] <= 0
END;

Answer 3

Attention! When you create a trigger by FOR INSERT or AFTER INSERT then don't write insert into table select * from inserted, because DB will insert data automatically, you can do only ROLLBACK this process. But, when creating a trigger by INSTEAD OF INSERT then you must write insert into table select * from inserted, else inserting not be doing.