Time interval calculation in time series using SQL - mysql

Question

I have a MySQL table like this
CREATE TABLE IF NOT EXISTS `vals` (
`DT` datetime NOT NULL,
`value` INT(11) NOT NULL,
PRIMARY KEY (`DT`)
);
the DT is unique date with time
data sample:
INSERT INTO `vals` (`DT`,`value`) VALUES
('2011-02-05 06:05:00', 300),
('2011-02-05 11:05:00', 250),
('2011-02-05 14:35:00', 145),
('2011-02-05 16:45:00', 100),
('2011-02-05 18:50:00', 125),
('2011-02-05 19:25:00', 100),
('2011-02-05 21:10:00', 125),
('2011-02-06 00:30:00', 150);
I need to get something like this:
start|end|value
NULL,'2011-02-05 06:05:00',300
'2011-02-05 06:05:00','2011-02-05 11:05:00',250
'2011-02-05 11:05:00','2011-02-05 14:35:00',145
'2011-02-05 14:35:00','2011-02-05 16:45:00',100
'2011-02-05 16:45:00','2011-02-05 18:50:00',125
'2011-02-05 18:50:00','2011-02-05 19:25:00',100
'2011-02-05 19:25:00','2011-02-05 21:10:00',125
'2011-02-05 21:10:00','2011-02-06 00:30:00',150
'2011-02-06 00:30:00',NULL,NULL
I tried the following query:
SELECT T1.DT AS `start`,T2.DT AS `stop`, T2.value AS value FROM (
SELECT DT FROM vals
) T1
LEFT JOIN (
SELECT DT,value FROM vals
) T2
ON T2.DT > T1.DT ORDER BY T1.DT ASC
but it returns to many rows (29 instead of 9) in result and I cold not find any way to limit this using SQL. Is it Possible in MySQL?

Answer 1

Use a subquery
SELECT
(
select max(T1.DT)
from vals T1
where T1.DT < T2.DT
) AS `start`,
T2.DT AS `stop`,
T2.value AS value
FROM vals T2
ORDER BY T2.DT ASC
You can also use a MySQL specific solution employing variables
SELECT CAST( #dt AS DATETIME ) AS `start` , #dt := DT AS `stop` , `value`
FROM (SELECT #dt := NULL) dt, vals
ORDER BY dt ASC
But you need to do it precisely
the ORDER by must be present otherwise the variables don't roll properly
the variable needs to be NULLified within the query using a subquery to set it, otherwise if you run it twice in a row, the 2nd time it will not start with NULL

Answer 2

You can use a server-side variable to simulate it:
select #myvar as start, end, value, #myvar := end as next_rows_start
from vals
Variables are interpreted from left-right in sequence, so the two references to #myvar (start and next_rows_start) will output with two different values.
Just remember to reset #myvar to null before and/or after the query, otherwise the second and subsequent runs will have a wrong first row:
select #myvar := null

Answer 3

This would be easier if the table had a running ID column which corresponds to the times in DT (same order). If you don't want to change the table you can use a temp:
drop table if exists temp;
CREATE TABLE temp (
`id` INT(11) AUTO_INCREMENT,
`DT` datetime NOT NULL,
`value` INT(11) NOT NULL,
PRIMARY KEY (`id`)
);
insert into temp (DT,value) select * from vals order by DT asc;
select t1.DT as `start`, t2.DT as `end`, t2.value
from temp t2
left join temp t1 ON t2.id = t1.id + 1;