>Hi,
>
>I am trying to create a trigger to update quantity on hand field of my stock items table whenever a transfer is done.
>
>This is my code:
>
>

USE [BPOptical]
>GO
>
>/****** Object:  Trigger [dbo].[Transfers_Insert_AdjustItemQty]    Script Date: 11/07/2013 02:17:31 PM ******/
>SET ANSI_NULLS ON
>GO
>
>SET QUOTED_IDENTIFIER ON
>GO
>
>
>-- =============================================
>-- Author:		<Author,,Name>
>-- Create date: <Create Date,,>
>-- Description:	<Description,,>
>-- =============================================
>CREATE TRIGGER [dbo].[Transfers_Insert_AdjustItemQty]
>   ON [dbo].[transfers]
>   AFTER INSERT
>AS 
>BEGIN
>	-- SET NOCOUNT ON added to prevent extra result sets from
>	-- interfering with SELECT statements.
>	SET NOCOUNT ON;
>
>	DECLARE @Quantity int
>	SET @Quantity = (SELECT trn_qty FROM inserted)
>
>	DECLARE @ItemId char(16)
>	SET @ItemId = (SELECT trn_itmfk FROM inserted)
>
>    -- Insert statements for trigger here
>	UPDATE dbo.Items
>			SET Items.iqtyonhand = Items.iqtyonhand - @Quantity
>		WHERE iitemid = @ItemId
>			AND iItemTypId = '43' -- 43 is for Inventory items
>
>
>END
>
>
>GO

>
>Is there something wrong with my logic here resulting in a null value?

There is one common mistake in this trigger as it's written to work only with a single insert.

I'll attempt to fix your code:

CREATE TRIGGER [dbo].[Transfers_Insert_AdjustItemQty]
   ON [dbo].[transfers]
   AFTER INSERT
AS 
BEGIN
	SET NOCOUNT ON;

                   MERGE dbo.Items as Target
                  USING (select trn_itmfk as ItemId, SUM(trn_Qty) as TotalQty 
FROM Inserted GROUP BY trn_itmfk) AS Source ON Target.ItemId = Source.ItemId

                 WHEN Matched  AND Target.ItemTypId  = '43' THEN UPDATE
                 SET iQtyOnHand = iQtyOnHand - COALESCE(Source.TotalQty,0);


END


GO

This is assuming you're using SQL Server 2008 and up.