SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE AddItems
(
Item_ID int IDENTITY PRIMARY KEY,
Quantity int NOT NULL,
Item_Code varchar(10) NOT NULL,
Item_Description varchar(100) NOT NULL
);

CREATE TABLE Products
(
Product_Id int IDENTITY PRIMARY KEY,
Item_Code varchar(10) NOT NULL,
Item_Description varchar(100) NOT NULL,
Available_Quantity int NOT NULL DEFAULT(0)
);
CREATE TRIGGER trg_AddItems_Insert
ON AddItems
FOR INSERT
AS
BEGIN
    
    /* update existing products */
    UPDATE   p
    SET      Available_Quantity = Available_Quantity + Qty
    FROM     Products p
    JOIN     (SELECT   Item_code, SUM(Quantity) Qty
              FROM     inserted
              GROUP BY Item_Code) s
    ON       p.Item_Code = s.Item_Code;

    
    /* insert new products */
    INSERT INTO Products
    (Item_Code, Item_Description, Available_Quantity)
    SELECT   i.Item_Code, i.Item_Description, Sum(Quantity)
    FROM     inserted i
    WHERE NOT EXISTS(SELECT 1 
                     FROM Products 
                     WHERE Item_Code = i.Item_Code)
    GROUP BY i.Item_Code, i.Item_Description;

END
INSERT INTO AddItems VALUES 
(10, 'AAA', 'PRODUCT A'),
(5,  'AAA', 'PRODUCT A'),
(7,  'CCC', 'PRODUCT C'),
(1,  'BBB', 'PRODUCT B');

INSERT INTO AddItems VALUES 
(25, 'AAA', 'PRODUCT A'),
(10, 'BBB', 'PRODUCT B'),
(3,  'CCC', 'PRODUCT C'),
(5,  'CCC', 'PRODUCT C');
14 rows affected

SELECT * FROM PRODUCTS;


Product_Id
Item_Code
Item_Description
Available_Quantity


1
AAA
PRODUCT A
40

2
BBB
PRODUCT B
11

3
CCC
PRODUCT C
15