SQL如何对指定行相减生成自定义列计算各级别独有用户数

我需要统计每个用户等级的总用户数，对比完成对应等级的总用户数，生成Unique Users列：该列取值为当前等级Users数减去下一级Users数，最高等级Appr无需减法，Unique Users值直接等于该等级的Users数，参考示例如下：

现有统计各等级总用户数的SQL代码 #

select 
cpnt.cpnt_id 
,count ( distinct pc.stud_id ) users
from 
pa_stud_program sp,
        pa_program p,
        pa_student s,
        pa_stud_cpnt pc,
        ps_program_type pt,
        pa_cpnt cpnt
WHERE p.PROGRAM_SYS_GUID = sp.PROGRAM_SYS_GUID
    and pc.compl_dte is not null
    and cpnt.cpnt_id in ('Trainee','SvcTech','CrewChief','SvcCoord','Appr')
    and s.jp_id in ('1801','1805','1810','1811')
    and s.EMP_STAT_ID = 'Active'
    AND cpnt.CPNT_TYP_ID     = p.CPNT_TYP_ID
    AND cpnt.CPNT_ID         = p.CPNT_ID
    AND cpnt.REV_DTE         = p.REV_DTE
    AND pc.STUD_ID           = sp.STUD_ID
    AND sp.stud_id           = s.STUD_ID
    AND pc.CPNT_ID           = sp.CPNT_ID
    AND pc.CPNT_TYP_ID       = sp.CPNT_TYP_ID
    AND pc.REV_DTE           = sp.REV_DTE
    AND pc.seq_num           = sp.seq_num
    AND pt.PROGRAM_TYPE_ID   = p.PROGRAM_TYPE   
    /** and s.PERSON_ID_EXTERNAL  in [UserSearch]*/ 
group by cpnt.cpnt_id
order by users desc

内容的提问来源于stack exchange，提问作者Mr Grimm