如何在oracle中使用sql获取不同值的结果

Question

我使用以下查询：

select MONITOR_ALERT_INSTANCE_ID as Id
     , description
     , created_date 
from monitor_alert_instance 
where description in(
                    select distinct(description)  
                    from monitor_alert_instance
                    where co_mod_asset_id=1223
                    group by description
                    )

，输出结果为：

如何根据最新（日期和时间）获得结果，该结果将在结果集中显示三个不同的值。预期的输出是：

+----------------------------------+---------------------------------------+--
| 766  |  Test.....                          |  14-03-04 14:56:51.000000000  |
| 765  |  Water_pH_sensor_malfunction        |  14-03-04 13:55:04.000000000  |
| 762  |  Water_turbidity_meter_malfunction  |  14-03-04 13:54:33.000000000  |
+----------------------------------+---------------------------------------+--

由于

Answer 1

您可以使用聚合获得您想要的结果：

select max(MONITOR_ALERT_INSTANCE_ID) as Id, description, max(created_date) as created_date
from monitor_alert_instance 
where description in (select description 
                      from monitor_alert_instance
                      where co_mod_asset_id = 1223
                     )
group by description;

请注意，我简化了子查询。使用distinct时group by是多余的。使用in时也不一定。

编辑：

我认为您可以使用此查询获得相同的结果：

select max(MONITOR_ALERT_INSTANCE_ID) as Id, description, max(created_date) as created_date
from monitor_alert_instance 
group by description
having max(case when co_mod_asset_id = 1223 then 1 else 0 end) = 1;

having子句确保说明适用于资产1223。

哪种表现更好取决于许多因素，但这可能比in版本表现更好。 （或者表格可能足够小，以至于任何性能差异都可以忽略不计。）

Answer 2

select id, 
       description,
       created_date
from (
  select MONITOR_ALERT_INSTANCE_ID as Id, 
         description
         created_date, 
         row_number() over (partition by description order by created_date desc) as rn
  from monitor_alert_instance 
  where co_mod_asset_id = 1223
) t
where rn = 1