Creating INSERT trigger that sets values to 0 - sql-server

Question

I have two tables : Invoice and Invoice_item, relationship 1 to many.
The Invoice_item table has columns Number_sold and Item_price, and the Invoice table has Number_sold_total and Item_price_total columns that will store total values of columns Number_sold and Item_price from the Invoice_item table with the same Invoice_ID key.
CREATE TABLE [Invoice] (
[Invoice_ID] [int] NOT NULL,
[Number_sold_total] [int] NOT NULL,
[Item_price_total] [decimal] NOT NULL,
PRIMARY KEY ([Invoice_ID]));
CREATE TABLE [Invoice_item] (
[Invoice_item_ID] [int] NOT NULL,
[Invoice_ID] [int] NOT NULL,
[Number_sold] [int] NOT NULL,
[Item_price] [decimal] NOT NULL,
PRIMARY KEY ([Invoice_item_ID],[Invoice_ID],
FOREIGN KEY ([Invoice_ID]) REFERENCES [Invoice]([Invoice_ID]);
So, if there are three rows in Invoice_item with the same Invoice_ID, the row with that Invoice_ID in Invoice table will have SUM values of corresponding columns in Invoice_item table.
Let's say i have three rows in Invoice_item table and columns Item_price with values 100,200 and 300, and they have the Invoice_ID = 3. The column Item_price_total in Invoice will have value of 600, where the Invoice_ID = 3.
QUESTION -
My task is to create an insert trigger on table Invoice that will set the values of Number_sold_total and Item_price_total to 0(ZERO) if there is no Invoice_item with corresponding Invoice_ID -> IF NOT EXISTS (Invoice.Invoice_ID = Invoice_item.Invoice_ID)...
I am using SQL Server 2017.

Answer 1

Ideally you would not implement this using triggers.
Instead you should use a view. If you are worried about querying performance, you can index it, at the cost of insert and delete performance.
CREATE VIEW dbo.Invoice_Totals
WITH SCHEMABINDING
AS
SELECT
i.Invoice_ID,
Number_sold = SUM(i.Number_sold),
Item_price = SUM(i.Item_price),
ItemCount = COUNT_BIG(*) -- must include count for indexed view
FROM dbo.Invoice_item;
And then index it
CREAT UNIQUE CLUSTERED INDEX CX_Invoice_Totals ON Invoice_Totals
(Invoice_ID);
If you really, really want to do this using triggers, you can use the following
CREATE OR ALTER TRIGGER TR_Invoice_Total
ON dbo.Invoice_item
AFTER INSERT, UPDATE, DELETE
AS
SET NOCOUNT ON; -- prevent spurious resultsets
IF (NOT EXISTS (SELECT 1 FROM inserted) AND NOT EXISTS (SELECT 1 FROM deleted))
RETURN; -- early bail-out if no rows
UPDATE i
SET Number_sold_total += totals.Number_sold_total,
Item_price_total += totals.Item_price_total
FROM Invoice i
JOIN (
SELECT
Invoice_ID = ISNULL(i.Invoice_ID, d.Invoice_ID),
Number_sold_total = SUM(ISNULL(i.Number_sold, 0) - ISNULL(d.Number_sold, 0)),
Item_price_total = SUM(ISNULL(i.Item_price, 0) - ISNULL(d.Item_price, 0))
FROM inserted i
FULL JOIN deleted d ON d.Invoice_ID = i.Invoice_ID
GROUP BY
ISNULL(i.Invoice_ID, d.Invoice_ID)
) totals
ON totals.Invoice_Id = i.Invoice_ID;
db<>fiddle
The steps of the trigger are as follows:
Bail out early if the modification affected 0 rows.
Join the inserted and deleted tables together on the primary key. This needs to be a full-join, because in an INSERT there are no deleted and in a DELETE there are no inserted rows.
Group up the changed rows by Invoice_ID, taking the sum of the differences.
Join back to the Invoice table
Update the Invoice table adding the total difference to each column.
This effectively recreates what the indexed view would do for you automatically.
You cannot just select the first row from inserted and deleted into variables, as there may be multiple rows affected. You must join and group them