Maria DB - update row with value of previous row + constant - mysql

Question

I have table called dobridol with several column.
CREATE TABLE IF NOT EXISTS `dobridol` (
`id` int(6) unsigned NOT NULL,
`dt` varchar(200) NOT NULL,
`p2` int(6) NOT NULL,
`p6` int(6) NOT NULL,
PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `dobridol` (`id`, `dt`, `p2`,`p6`) VALUES
('1', '2021-02-28 23:50:00', '100', '600'),
('2', '2021-02-28 23:55:00', '200', '700'),
('3', '2021-03-01 00:00:00', '300', '800'),
('4', '2021-03-01 00:05:00', '400', '900'),
('5', '2021-03-01 00:10:00', '400', '900'),
('6', '2021-03-01 00:15:00', '400', '900'),
('7', '2021-03-01 00:20:00', '500', '1000'),
('8', '2021-03-01 00:25:00', '600', '1100');
The table has values for January and March also.
I want to be able to UPDATE table like that:
I select period as month, then I add constant value (in this case I added 39) to p6 ONLY WHEN p2 value is different than previous row p2.
If this is the case I have to add 39 to PREVIOUS row p6 value.
update dobridol join
(select tt.*,
sum(case when p2 <> prev_p2 then 1 else 0 end) over (order by dt) as cnt
from (select tt.*,
lag(p2) over (order by dt) as prev_p2
from dobridol tt
) tt
) tt
on tt.id = dobridol.id
set dobridol.p6 = cnt * 39 + <PREVIOUS_ROW_p6_VALUE_HAS_TO_BE_HERE>
where cnt > 0
The query should look-like this but I have to replace this <PREVIOUS_ROW_VALUE_HAS_TO_BE_HERE> with the right syntax of picking last row p6. How can I pick it?
Also where to add clause
dobridol.dt BETWEEN '2021-03-01' AND '2021-03-30'
in the SQL query?

Answer 1

If I understand correctly, you can use lag():
update dobridol join
(select tt.*,
lag(p6) over (order by dt) as prev_p6,
sum(case when p2 <> prev_p2 then 1 else 0 end) over (order by dt) as cnt
from (select tt.*,
lag(p2) over (order by dt) as prev_p2
from dobridol tt
) tt
) tt
on tt.id = dobridol.id
set dobridol.p6 = tt.cnt * 39 + tt.prev_p6
where cnt > 0 ;
Here is a db<>fiddle.