Help Automatically Populating Fields base on PK

[ITHELP85]

using SQL Server 2005

I have a table SALES_ITEM, users should be able to input the Primary Keys (ItemNumberSK and InvoiceNumber), and Qty. I want the ItemName and UnitPrice fields to update automatically from the WAREHOUSE table.
I wrote a trigger already but something is wrong with it as I get this error (My code is listed under the error):

---------------------------
Microsoft SQL Server Management Studio
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No row was updated.

The data in row 1 was not committed.
Error Source: .Net SqlClient Data Provider.
Error Message: Cannot insert the value NULL into column 'InvoiceNumberS K', table 'prac.dbo.SALES _ITEM'; column does not allow nulls. INSERT fails.

The statement has been terminated.

Correct the errors and retry or press ESC to cancel the change(s).
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OK Help
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/* Creates SALES_ITEM table */

CREATE TABLE SALES_ITEM(
InvoiceNumberSK int NOT NULL,
ItemNumberSK int NOT NULL,
ItemName char(50) NULL,
Qty numeric(7, 0) NOT NULL,
UnitPrice money NULL,
ExtendedPrice AS CASE
WHEN UnitPrice > 0 AND Qty > 0 Then UnitPrice * Qty
END

CONSTRAINT Sales_ItemPK PRIMARY KEY (InvoiceNumberS K, ItemNumberSK),
CONSTRAINT Sales_ItemInvoi ceNumberFK FOREIGN KEY (InvoiceNumberS K) REFERENCES SALES (InvoiceNumberS K),
CONSTRAINT Sales_ItemItemN umberSKFK FOREIGN KEY (ItemNumberSK) REFERENCES WAREHOUSE (ItemNumberSK)
);

/* Creates WAREHOUSE table */

CREATE TABLE WAREHOUSE(
ItemNumberSK int NOT NULL,
ItemName char(50) NULL,
ItemUnitPrice money NULL,
ItemQty numeric(7, 0) NULL,

CONSTRAINT WarehousePK PRIMARY KEY (ItemNumberSK),
CONSTRAINT WarehouseFK FOREIGN KEY (ItemNumberSK) REFERENCES ITEM_PURCHASE (ItemNumberSK)
);




/* Creates Trigger that Automatically Inserts ItemName & UnitPrice From WAREHOUSE table, with the matching ItemNumberSK */

go
CREATE TRIGGER Populate_ItemNa me_UnitPrice ON SALES_ITEM AFTER INSERT
AS
DECLARE
@ItemNumberSK int,
@ItemName char(50),
@UnitPrice money


SELECT @ItemNumberSK = ItemNumberSK
FROM WAREHOUSE


BEGIN
INSERT INTO SALES_ITEM (ItemName, UnitPrice)
VALUES (@ItemName, @UnitPrice)

END

[ck9663]

Removed unnecessary quote.

You never store any value in @ItemName and @UnitPrice

	INSERT INTO SALES_ITEM (ItemName, UnitPrice)
	VALUES (@ItemName, @UnitPrice)

-- CK