SQL Join one to many 从满足条件的相同键值组中提取值

Question

我有两个 table 我想加入一对多关系。 parent table 具有与 child table 共享的唯一键值。 child table 保留该键的历史记录，因此有多个记录。示例如下。

我需要的是提取一个警报 ID 的工作人员名称，其中对于具有相同警报 ID 的给定组，操作是 'Alert Closed'。如果该组相同的警报 ID 没有带有 action = 'Alert Closed' 的记录，则采用具有最新时间戳的工作人员名称。我缺乏关于如何比较时间戳列的知识，当 'Alert Closed' 为真时我仍然得到重复项。

Table 1

alert_id            
---------
123         
456         
537

Table 2

alert_id    worker_name action          timestamp
---------------------------------------------------------
123         system      Alert Created   8/6/2016  8:05:26 
123         james bond  Alert Opened    8/6/2016  8:05:30
123         james bond  Alert Closed    8/6/2016  8:05:35
123         james bond  Record updated  8/6/2016  8:05:35
456         system      Alert Created   8/6/2016  8:05:26
456         admin       Alert updated   8/6/2016  8:06:14
537         system      alert created   8/6/2016  8:07:20
537         Mary hill   Alert Closed    8/6/2016  8:08:26

结果 table 应该是：

alert_id    worker_name     
-----------------------
123         james bond      
456         admin       
537         Mary Hill

Answer 1

这是一个优先查询。您可以使用 row_number() 来接近它。诀窍是正确排序：

select t2.*
from (select t2.*,
             row_number() over (partition by alert_id
                                order by (case when action = 'Alert Closed' then 1 else 2 end),
                                         timestamp desc
                               ) as seqnum
      from t2
     ) t2
where seqnum = 1;

SQL Join one to many 从满足条件的相同键值组中提取值

SQL Join one to many extract value from group of same key value where condition is met

sql

join

one-to-many

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