This has been solved in other posts and seems to be a very common issue.

Here's one solution (I added DDL and sample data in an usable format and you should try to do this for your next posts, as it will give you better and faster answers).

DECLARE @Tabletable(

idint,

device int,

value int)

INSERT @Table

SELECT 9, 456, 70 UNION ALL

SELECT 8, 456, 60 UNION ALL

SELECT 7, 123, 70 UNION ALL

SELECT 6, 123, 60 UNION ALL

SELECT 5, 456, 50 UNION ALL

SELECT 4, 456, 10 UNION ALL

SELECT 3, 123, 50 UNION ALL

SELECT 2, 123, 20 UNION ALL

SELECT 1, 123, 10 ;

WITH CTE AS (

SELECT id,

device,

value,

ROW_NUMBER() OVER(ORDER BY device, id) rownum

FROM @Table)

SELECT a.id

FROM CTE a

JOIN CTE b ON a.rownum = b.rownum + 1

WHERE a.value - b.value > 20

EDIT: See what I meant when saying this is only one solution? Now you have three different ones but one could have problems with missing ids. And mine had probably a problem with the order (corrected now).