Select previous data entry by datetime - mysql

Question

I have a mysql database with a table with performance data. Each data table entry is for a specific counter.
DATA
----
dat_date BIGINT
value BIGINT
dat_date DATETIME(3)
FK_dt_id BIGINT
For each entries I need to compute the speed and acceleration. So I want to create a SELECT that will select a row with it PREVIOUS row.
FK_dt_id is a foreigh key on a counter type
I've try this :
SELECT d1.dat_date,
d1.value v1,
d1.PK_dat_id
FROM data d1
INNER JOIN (SELECT * FROM data d2 ORDER BY d2.dat_date) d2
ON (d2.dat_date < d1.dat_date
AND d2.FK_dt_id = d1.FK_dt_id)
It works but I can't get the previous primary key, only it's value.
Any ideas?

Answer 1

The fastest (not necesarily the easiest) way to do things like this (with "pure" MySQL syntax) is with a little trick using user variables:
select #pk_data_id as prev_pk_data_id
, #pk_data_id := (case
when #fk_counter_id = fk_counter_id then a.pk_data_id
else 0
end) as pk_data_id
, #fk_counter_id := a.fk_counter_id as fk_counter_id
from
(select #pk_data_id := 0, #fk_counter_id := 0) as init
, data as a
order by a.fk_counter_id, a.pk_data_id
Once you have this rowset, you can join it with your data table.
I'd put this result in a temporary table and use it later; something like this:
drop table if exists temp_tbl;
create temporary table temp_tbl
select #pk_data_id as prev_pk_data_id
, #pk_data_id := (case
when #fk_counter_id = fk_counter_id then a.pk_data_id
else 0
end) as pk_data_id
, #fk_counter_id := a.fk_counter_id as fk_counter_id
from
(select #pk_data_id := 0, #fk_counter_id := 0) as init
, data as a
order by a.fk_counter_id, a.pk_data_id;
alter table temp_tbl
add index dId (pk_data_id),
add index pdId (prev_pk_data_id),
add index cId (fk_counter_id);
-- Now use the temp table to get what you need
select d1.*
, d2.pk_data_id as prev_pk_data_id
, d2.data_value as prev_data_value
, d2.data_datetime as prev_data_datetime
from data as d1
-- If you don't use the temp table, substitute 'temp_tbl' with
-- the query from above
inner join temp_tbl as a
on d1.pk_data_id = a.pk_data_id
and d1.fk_counter_id = a.fk_counter_id
left join data as d2
on a.prev_pk_data_id = d2.pk_data_id
and a.fk_counter_id = d2.fk_counter_id