選擇返回鎖定的使用者或空行的預設值

我對 Oracle 數據庫知之甚少，並從這裡使用了一些問題來獲取此查詢：

select case when ((sysdate - timestamp)*24 < 1 and returncode <> 0) then 

(username  || '@' || userhost|| ' ON ' || to_char(timestamp,'DD-MON-YYYY HH24:MI:SS')|| ' FAILED AT ' || action_name || ' BECAUSE ' || returncode || decode(returncode,'28000',':Maximum Login Attempt','1004',':Wrong Connection','1005',':NULL Password','1017',':Wrong Password','1045',':Insufficient Priviledge','0',':Login Accepted',':Unkown Code')) 

else 'none' 

end as "locked users" from sys.dba_audit_session where (sysdate - timestamp)*24 < 1 and returncode <> 0 order by timestamp;

使用此查詢，我想檢索鎖定在哪個日期的使用者以及導致鎖定的原因，如果沒有使用者，則僅返回“無”而不是“未選擇行”。對於鎖定的使用者，它可以工作，但 else 語句不能。有人可以解釋我為什麼嗎？我需要用 ODBC 修復我的 zabbix 數據庫監控。

如果原始查詢不返回任何行，只需選擇查詢的並集和“無”常量：

with s as (
select case when ((sysdate - timestamp)*24 < 1 and returncode <> 0) then 
(username  || '@' || userhost|| ' ON ' || to_char(timestamp,'DD-MON-YYYY HH24:MI:SS')|| ' FAILED AT ' || action_name || ' BECAUSE ' || returncode || decode(returncode,'28000',':Maximum Login Attempt','1004',':Wrong Connection','1005',':NULL Password','1017',':Wrong Password','1045',':Insufficient Priviledge','0',':Login Accepted',':Unkown Code')) 
else 'none' 
end as "locked users" from sys.dba_audit_session where (sysdate - timestamp)*24 < 1 and returncode <> 0 order by timestamp
)
select 'none' as "locked_users" from dual where not exists (select * from s)
union all
select * from s;

引用自：https://dba.stackexchange.com/questions/202827