Ecart enregistrement précédent

SELECT t1.ID, t1.lg, max(t2.ID) AS previd
FROM lgtb AS t1 LEFT JOIN lgtb AS t2 ON t2.ID < t1.ID
GROUP BY t1.ID, t1.lg;

SELECT q.lg as lg, q.lg - t.lg as ecart
FROM  lgqprevid as q left JOIN  lgtb as t ON t.ID = q.previd;

SELECT q.lg AS lg, q.lg - t.lg AS ecart
FROM (SELECT t1.ID, t1.lg, max(t2.ID) AS previd
FROM lgtb AS t1 LEFT JOIN lgtb AS t2 ON t2.ID < t1.ID
GROUP BY t1.ID, t1.lg)  AS q LEFT JOIN lgtb AS t ON t.ID = q.previd;