SQl trigger after insert error - sql

Question

I'm trying to update/insert a value after I inserted a new row into my SQL table. I'm using a trigger to accomplish this but I get an error which I do understand what it means but I don't know how to fix it. I was under the impression that I could get the inserted row from the inserted table based on the #id I declared, that part seems to work but why do I get the error that I'm not allowed to enter a NULL value into my "invoicenumber" field (I know it's the PK but I don't wont to enter anything into this field (it's already filled with correct data from the insert itself).
So, can anyone explain to me what I did wrong?
The table I used:
table delivery_invoice
id - int
invoicenumber(PK) - varchar(50)
amount6 - money
amount21 - money
distributor - int
payed - char(3)
deliverycosts - money
deliverydate - datetime
deliverytime - datetime
VATamount6 - calculated field
VATamount21 - calculated field
VATamountdelivery - int
The trigger I used
USE TestDelivery
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author: James
-- Create date: 11-08-2017
-- Description: change VATamountdelivery field after a new row is inserted
-- =============================================
CREATE TRIGGER trgVATAmountDelivery
ON dbo.delivery_invoice
AFTER INSERT
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
Declare #id int
Declare #amount money
Declare #invoice varchar(50)
Select #id = id from inserted
Select #amount = VATamount21 from inserted
IF #amount > 0
insert into dbo.delivery_invoice(VATamountdelivery)
values(21)
else
insert into dbo.delivery_invoice(VATamountdelivery)
values(6)
END
GO
And last but not least the error I got:
Error. Cannot insert the value NULL into column 'invoicenumber', table
'TestDelivery delivery_invoice'; column does not allow nulls. INSERT fails.
The statement has been terminated.
Any help would be appriciated!
J.

Answer 1

You don't want to insert a new row with the value. I think you just want to change the value.
However, your trigger has other issues. In particular, never assume that inserted contains only one row. That is just a bug waiting to happen.
CREATE TRIGGER trgVATAmountDelivery ON dbo.delivery_invoice
AFTER INSERT
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
update di
set VATamountdelivery = (case when i.VATamount21 > 0 then 21 else 6 end)
from dbo.delivery_invoice di join
inserted i
on i.id = di.id
END;