SELECT t_a.job_id, t_a.max_lvl, ISNULL(max_lvl - ISNULL ((SELECT max_lvl FROM jobs WHERE job_id = (SELECT MAX(job_id) FROM jobs WHERE job_id < t_a.job_id)), 0), 0) AS 差额FROM jobs t_a
pubs数据库jobs表,job_id为int型
