oracle decode函數的用法
阿新 • • 發佈:2018-04-12
jaoracle decode函數,不管查詢的結果是否滿足‘PRIMARY’,參數中的select語句還是會執行的,通過查看執行計劃就知道
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$database;
decode函數等價於cash when語句,只不過當條件比較多時,decode函數書寫起來比較簡潔一點而已
select case
when database_role = ‘PRIMARY‘ then (select tname from tab where rownum=1)
else ‘0‘
end as retvalue
from v$database;
oracle decode函數的用法