mysql difference update

DROP table testtbl 
;
CREATE table
testtbl (id int,
dat datetime,
valuenum int,
valuenum2 int,
value varchar(30),
primary key(id,
dat))
;
;
insert into testtbl values(7, '1990-01-01', 2,-1, 'abc');
insert	into testtbl values(7, '1990-01-02', 16,-1, 'abc');
insert	into testtbl values(7, '1990-01-03', 3,-1, 'abc');
insert	into testtbl values(8, '1990-01-03', 35,-1, 'abc');

insert into testtbl values(9, '1990-01-02', 7,-1, 'def');
insert into testtbl values(9, '1990-01-03', 1,-1, 'def');
insert into testtbl values(9, '1990-01-04', 4,-1, 'def');

;
SELECT *
FROM testtbl t 
;
;

UPDATE
	testtbl t
inner join (
	SELECT
		x.id,
		MIN(x.dat) datsecond,
		MAX(x.dat) datfirst
	from
		(
		SELECT
			cur.id,
			cur.dat,
			COUNT(nxt.id) rownum
		from
			testtbl cur
		left join testtbl nxt
on
			nxt.id = cur.id
			and nxt.dat >= cur.dat
		group by
			cur.id,
			cur.dat 
) x
	WHERE
		x.rownum <= 2
	group by
		x.id
) tt
ON
	tt.id = t.id
	and tt.datfirst = t.dat
left join testtbl t2 
on
	tt.datfirst > tt.datsecond
	and t2.id = tt.id
	and t2.dat = tt.datsecond
set
	t.valuenum2 = t.valuenum -
	case
		when t2.valuenum is null then 0
		else
	t2.valuenum -
		case
			when t.valuenum >= t2.valuenum then 0
			else 100
		end
	end