纵表转横表

 原表

select tt.*
from
  (select
  t.city,
  to_char(t.oper_date,'yyyy') 年度,
   sum(
   decode(t.status,'优',1,'良',1,0)
   ) 优良天数,
   sum(
    decode(t.grade,'Ⅰ',1,0)
   ) 一级天数,
   sum(
    decode(t.grade,'Ⅱ',1,0)
   ) 二级天数
  from 
      city_day t  
  group by to_char(t.oper_date,'yyyy'),t.city)tt
where tt.年度 = to_char(sysdate,'yyyy')-1 
or
    tt.年度 = to_char(sysdate,'yyyy')-2
order by tt.年度

转后

select ttt.city,
sum(decode(ttt.年度,to_char(sysdate,'yyyy'),ttt.优良天数,null)) 今年优良天数,
sum(decode(ttt.年度,to_char(sysdate,'yyyy'),ttt.一级天数,null)) 今年一级天数,
sum(decode(ttt.年度,to_char(sysdate,'yyyy'),ttt.二级天数,null)) 今年二级天数,
sum(decode(ttt.年度,to_char(sysdate,'yyyy')-1,ttt.优良天数,null)) 去年优良天数,
sum(decode(ttt.年度,to_char(sysdate,'yyyy')-1,ttt.一级天数,null)) 去年一级天数,
sum(decode(ttt.年度,to_char(sysdate,'yyyy')-1,ttt.二级天数,null)) 去年二级天数
from (select tt.*
      from
        (select
        t.city,
        to_char(t.oper_date,'yyyy') 年度,
         sum(
         decode(t.status,'优',1,'良',1,0)
         ) 优良天数,
         sum(
          decode(t.grade,'Ⅰ',1,0)
         ) 一级天数,
         sum(
          decode(t.grade,'Ⅱ',1,0)
         ) 二级天数
        from 
            city_day t  
        group by to_char(t.oper_date,'yyyy'),t.city)tt
      where tt.年度 = to_char(sysdate,'yyyy')
      or
          tt.年度 = to_char(sysdate,'yyyy')-1
      order by tt.年度
     )ttt
group by city