CREATE TABLE [dbo].[Inventory](
[ID] [int] IDENTITY(1,1) NOT NULL
[SerialNo] [nvarchar](50) NOT NULL,
[Quantity] [decimal](12, 3) NULL)

CREATE TABLE [dbo].[InventoryDetails](
[ID] [int] IDENTITY(1,1) NOT NULL,
[InventoryID] [int] NOT NULL,
[ProductID] [int] NOT NULL,
[Quantity] [decimal](10, 3) NOT NULL)

Update I set Quantity=Sum(ID.Quantity)
from Inventory I
Inner Join inserted ID on I.ID=ID.InventoryID