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)