这是CTAIS中查询一般纳税人应申报数的SQL语句，有没有高手可以逐行就语法问题解释一下      
    substr(nsr_swjg_dm,1,5),count(*)     
from   Sb_Yzcwsbqc  a     
where  to_char(a.nd)=2006      
  and  to_char(a.yf)=7
  and a.zsxm_dm=01
  and a.nsrsbh in(select distinct (nsrsbh)
  from rd_nsrzg_lsxx b
 where b.nsrzg_dm in ('01', '02', '03')
   and exists (select nsrsbh
          from rd_nsrzg c
         where c.nsrsbh = b.nsrsbh
           and c.nsrzg_dm in ('01', '02', '03')
           and c.yxq_q < to_date('20060701', 'yyyymmdd')
           and c.wspzxh=b.wspzxh)
           or( not exists (select nsrsbh
          from rd_nsrzg c
         where c.nsrsbh = b.nsrsbh
           and c.nsrzg_dm in ('01', '02', '03')
           and c.yxq_q < to_date('20060701', 'yyyymmdd')
           and c.wspzxh=b.wspzxh)
           and b.yxq_z=to_date('20060630','yyyymmdd')))
group by  substr(nsr_swjg_dm,1,5)