oracle decode函数，不管查询的结果是否满足‘PRIMARY’，参数中的select语句还是会执行的,通过查看执行计划就知道

set autotrace traceonly

select decode(database_role,'PRIMARY',(select
trunc((size_MB-free_MB)*100/maxsize_MB) "retvalue"
from (
select
tablespace_name,case when sum(decode(autoextensible,'YES',1,0))>0 then 'YES'
else 'NO' end ext,trunc(sum(bytes)/1024/1024) SIZE_MB,trunc(sum(case when autoextensible='YES' then maxbytes else bytes
end)/1024/1024) MAXSIZE_MB
from dba_data_files
group by tablespace_name
) t1,(
select tablespace_name,trunc(sum(bytes)/1024/1024) FREE_MB
from dba_free_space
group by tablespace_name
) t2,(select tablespace_name
from dba_tablespaces
where contents='PERMANENT') t3
where t1.tablespace_name=t2.tablespace_name and t3.tablespace_name=t1.tablespace_name and t3.tablespace_name='SYSTEM'),0) as retvalue from v$database;

decode函数等价于cash when语句，只不过当条件比较多时，decode函数书写起来比较简洁一点而已

select case 
           when database_role  = 'PRIMARY' then  (select tname from tab where rownum=1)
           else '0' 
       end as retvalue 
from  v$database;