Oracle 从行到列中选择计数/值

Question

我一直在尝试从行中获取一些结果以呈现到列中。

桌子就像，

Event_ID
User ID
Event_Type (Call,App)
...

一个用户可以有多个事件类型，呼叫或约会。我想将结果列为

User_ID      Calls_Count      Appointments_Count
01            4                10
02            0                12

我已设法编写查询以获得所需的结果，但如果用户没有呼叫或约会，则记录不会出现在结果中。

select 
distinct(e.user_id),
Call.cc as Call_Count,
App.ac as App_Count

from events e,
(select user_id,event_type,count(user_id) as cc from events c where event_type = 'Call' group by user_id,event_type) Call,
(select user_id,event_type,count(user_id) as ac from events a where event_type = 'App' group by user_id,event_type) App

where e.user_id = Call.user_id
and e.user_id = App.user_id

order by user_id asc
;

如何将此查询转换为使用join，以便它返回所需的结果，或者有更好的方法来实现相同的结果。

在此先感谢

Answer 1

Oracle 11 版本：

select * from events 
  pivot (count(1) for (event_type) in ('App' Apps, 'Call' Calls))
  order by user_id

旧版本：

select user_id,
    count(case when event_type = 'App' then 1 end) Apps,
    count(case when event_type = 'Call' then 1 end) Calls
  from events 
  group by user_id
  order by user_id

---

编辑：对于pivot 的解决方案，最好将列限制为我们最初感兴趣的列，如下所示：

select * from (select user_id, event_type from events)
  pivot (count(1) for (event_type) in ('App' Apps, 'Call' Calls))
  order by user_id

SQLFiddle