Question

当我在查询的ELSE部分尝试使用listagg选择语句时，出现 ORA-00937 错误。有什么方法吗？

select 
CASE 
    when count(*) = 0 then 
        'There are no users connected' 
    else 
        (select 
        listagg(osuser, ', ') within group (order by osuser) 
        from (select distinct osuser from v$session) 
        where osuser != 'SYSTEM' and osuser not like 'VMCONFTEST%')
end 
from v$session
where username is not null 
and osuser not like 'VMCONFTEST%';

即使我用更简单的东西替换select语句，例如select osuser from v$session，我也会得到同样的错误。

Answer 1

采取略有不同的方法但似乎有效。不要套管并进行计数，只需检查聚合是否为空（coalesce返回系列中的第一个非空值），以及它是否替代了您的消息。这避免了我认为不需要的第二级分组。

太糟糕的listagg也不支持聚合中的不同;我们可以避免内联视图。

SELECT coalesce(listagg(A.osuser, ', ') within group (order by A.osuser), 
                'There are no users connected') as userList
FROM (select distinct osuser from v$session) A
WHERE A.osuser!= 'SYSTEM' and A.osuser not like 'VMCONFTEST%'

这确实存在开销，因为它试图生成您的case语句可能试图短路的用户列表。但是，如果V $ session中没有记录，则select distinct应该很快。

虽然说实话但我不确定为什么我们需要这样做。列表中的空值通常是足够的响应，表示没有用户。 UI将处理null意味着没有用户。

如果我们更多的内联视图的where子句，可能会更快..

SELECT coalesce(listagg(A.osuser, ', ') within group (order by A.osuser), 
                'There are no users connected') as userList
FROM (SELECT distinct osuser 
      FROM v$session
      WHERE A.osuser!= 'SYSTEM' 
        and A.osuser not like 'VMCONFTEST%') A

Answer 2

您的陈述缺少group by子句，例如下面有效：

select distinct(
CASE 
when count(*) = 0 then 
    'There are no users connected' 
else 
    (select 
    listagg(osuser, ', ') within group (order by osuser) 
    from (select distinct osuser from v$session) 
    where osuser != 'SYSTEM' and osuser not like 'VMCONFTEST%')
end)
from v$session
where username is not null 
and osuser not like 'VMCONFTEST%'
GROUP BY osuser;

Answer 3

select 
CASE 
   when cnt = 0 then 
    'There are no users connected' 
   else 
    (select 
     listagg(osuser, ', ') within group (order by osuser) 
     from (select distinct osuser from v$session) t
    where osuser != 'SYSTEM' and t.osuser not like 'VMCONFTEST%')
end result
from
 (select count(*) as cnt
 from v$session
 where username is not null 
and osuser not like 'VMCONFTEST%');

Answer 4

with conn_users as 
     (select count(*) conn_users
      from v$session
      where osuser != 'SYSTEM' 
        and osuser not like 'VMCONFTEST%'),
  aggr_users as       
     (select listagg(osuser, ', ') within group (order by osuser) 
        from (select distinct osuser from v$session) 
        where osuser != 'SYSTEM' and osuser not like 'VMCONFTEST%')
select 
CASE 
    when (select * from conn_users)  = 0 then 
        'There are no users connected' 
    else 
      (select * from aggr_users)
end 
from dual;

问题是无法忍受的计数

在我需要执行复杂子查询时，我喜欢使用with子句可能这个查询可以改进集中条件（重复）

在＆＃34; ELSE＆＃34;中使用select CASE语句给我ORA-00937：不是单组组功能

4 个答案: