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', (selecttrunc((size_MB-free_MB)*100/maxsize_MB) "retvalue"from (selecttablespace_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 bytesend)/1024/1024) MAXSIZE_MBfrom dba_data_filesgroup by tablespace_name) t1,(select tablespace_name,trunc(sum(bytes)/1024/1024) FREE_MBfrom dba_free_spacegroup by tablespace_name) t2,(select tablespace_namefrom dba_tablespaceswhere contents='PERMANENT') t3where 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;