oracle decode函数什么意思,oracle decode函数的用法

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;