You can also add PRIMARY KEY to the table definititon

DECLARE @item_table TABLE (primary_key INT IDENTITY(1,1) NOT NULL PRIMARY KEY,

                                    item_category_id INT,

                                    order_id INT )

The followings SELECT are better

SELECT @item_category_id = item_category_id ,@order_id = order_id

FROM @item_table

WHERE primary_key = @item_category_counter