Trigger to auto-increment ID based on certain criteria in TSQL - sql-server

Question

I would like to write a trigger so that when a new record comes in, if certain columns of it contains the same content as the previous record, one particular ID column for all existing entries would increase by 1 and the new entry would be 0, and if there is no existing record, the new record is automatically assigned to have ID 0 in that column.
For example, say my existing data is like this:
FirstName LastName InvoiceID Amt
-----------------------------------
John Doe 1 $5
Bill Jane 0 $6
John Doe 0 $3
Now say I want to insert John Doe with $3.5 of invoice, then the record would automatically becomes:
FirstName LastName InvoiceID Amt
-----------------------------------
John Doe 1 $5
Bill Jane 0 $6
John Doe 2 $3
John Doe 0 $3.5
Now if I need to insert someone new, say Jane Smith for $2, it would become:
FirstName LastName InvoiceID Amt
--------------------------------------
John Doe 1 $5
Bill Jane 0 $6
John Doe 2 $3
John Doe 0 $3.5
Jane Smith 0 $2
May I know how should write such trigger in T-SQL?
Thanks!

Answer 1

Here's what I would do, rather than having the trigger:
create table T (TID int IDENTITY(1,1) not null,FirstName varchar(10) not null,LastName varchar(10) not null,Amt decimal(18,4) not null)
go
create view V
as
select
FirstName,
LastName,
ROW_NUMBER() OVER (PARTITION BY FirstName,LastName ORDER BY TID desc) - 1 as InvoiceID,
Amt
from
T
go
And then set up your initial data:
insert into T(FirstName,LastName,Amt) values
('John','Doe',5),
('Bill','Jane',6)
go
insert into T(FirstName,LastName,Amt) values
('John','Doe',3)
And the new John Doe row:
insert into T(FirstName,LastName,Amt) values
('John','Doe',3.5)
And finally the Jane Smith row:
insert into T(FirstName,LastName,Amt) values
('Jane','Smith',2)
And then select from the view:
FirstName LastName InvoiceID Amt
---------- ---------- -------------------- ---------------------------------------
Bill Jane 0 6.0000
Jane Smith 0 2.0000
John Doe 0 3.5000
John Doe 1 3.0000
John Doe 2 5.0000
And so we have no concern that InvoiceID might be incorrect because we never store it.
select * from V