vælg de to seneste rækker for hver bruger for at få forskellen på en kolonne (MySQL)

Du skal forbedre dig på dette, men det er i bund og grund sådan, du gør det her. du kan bruge en selvtilmelding

SELECT uid, MAX(myvalue)-MIN(myvalue) AS diff FROM
    (SELECT a.* FROM tablename AS a
        LEFT JOIN tablename AS a2
            ON a.uid=a2.uid AND a.myvalue<=a2.myvalue
        GROUP BY uid,myvalue
        HAVING COUNT(*)<=2
    ) a
GROUP BY uid