好复杂的查询，谁能帮详细解释一下呢？？

nmgzw

老是看见这类查询语句的写法，但就是不明白，谁能给加个说明呢？非常感谢！

1. 
   
2. select *
   
3.   from (
   
4. select t1.trans_id,
   
5.   decode( r, 1, 'C1', 2, 'C2', 3, 'C3' ) cname,
   
6.   decode( r, 1, to_char(t1.c1,'dd-mon-yyyy hh24:mi:ss'),
   
7.              2, to_char(t1.c2),
   
8.              3, t1.c3) v1,
   
9.   decode( r, 1, to_char(t2.c1,'dd-mon-yyyy hh24:mi:ss'),
   
10.              2, to_char(t2.c2),
    
11.              3, t2.c3) v2
    
12. from
    
13.      (select rownum rr, trans.* from trans where trans_id = :bv ) t1,
    
14.      (select rownum rr, trans_tmp.* from trans_tmp where trans_id = :bv) t2,
    
15.      (select rownum r from all_objects where rownum <= 3 ) t3,
    
16.      (select rownum rr from dual) t4
    
17. where t1.rr(+) = t4.rr
    
18.   and t2.rr(+) = t4.rr
    
19.   )
    
20. where v1 <>; v2
    
21.    or (v1 is null and v2 is not null)
    
22.    or (v1 is not null and v2 is null)
    
23. /
    
24. 
    

复制代码

doni

挺简单的SQL