MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

SELECT @@VERSION


@@VERSION


8.0.27


CREATE TABLE tab AS 
SELECT 1 AS t, 'G' AS company, 40 AS quote
UNION SELECT 2 , 'G',    60 
UNION SELECT 3 , 'S',    60 
UNION SELECT 4,  'S',    20 
SELECT DISTINCT company, quote , LAG(quote) OVER(PARTITION BY company ORDER BY t)
FROM tab;



company
quote
LAG(quote) OVER(PARTITION BY company ORDER BY t)


G
40
null

G
60
40

S
60
null

S
20
60


SELECT DISTINCT company, quote - LAG(quote) OVER(PARTITION BY company ORDER BY t)
FROM tab;


company
quote - LAG(quote) OVER(PARTITION BY company ORDER BY t)


G
null

G
20

S
null

S
-40