基于dual 有意思的decode和case-when-then-else-end的测试

select  decode( op
                ,'a','hht,I am A; b is the base! and It is not a lie , '
                     ||'please believe me;I promise!'
                ,'b','hht,I am B; a is the base! and It is not a lie , '
                     ||'please believe me;I promise!'
                ,'c','I am not allow to tell you anything, sorry-ing......'
                ,'none') dcde
from (
     select case in_op
            when 'a' then 'b'
            when 'b' then 'a'
            else 'c'
            end op
             from (
                  select 'a' in_op
                  from dual
                  )
     )