-- prepare test data
declare	@t table (Payment_point varchar(5), Payment_type varchar(4), [month] datetime, amount smallint)

set dateformat dmy

insert	@t
select	'AA000', 'FLAG', '1/04/2006', 110 union all
select	'AA000', 'FLAG', '1/05/2006', 100 union all
select	'AA000', 'FLAG', '1/06/2006', 100 union all
select	'AA000', 'FLAG', '1/07/2006', 110 union all
select	'AA000', 'US', '1/01/2005', 600 union all
select	'AA000', 'US', '1/02/2005', 300 union all
select	'AA000', 'BON', '1/03/2005', 20 union all
select	'AA000', 'BON', '1/04/2005', 30

-- do the work
select		t.Payment_point,
		t.Payment_type,
		t.[month],
		t.amount,
		CASE
			WHEN v.amount IS NULL THEN 'No previous month'
			else str(100.0 * (t.[amount] - v.[amount]) / v.[amount], 17, 2)
		end '%_change_prev_month'
from		@t t
left join	@t v on v.payment_point = t.payment_point
			and v.payment_type = t.payment_type
			and year(v.[month]) = year(t.[month])
			and month(v.[month]) = month(t.[month]) - 1
order by	t.Payment_point,
		t.Payment_type,
		t.[month]