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