这是我现在唯一想要的场景
对于您显示的具有精确日期匹配的数据，您可以使用外部联接和合并来在存在匹配时使用分段值，并使用distinct来删除重复项，然后为第二个分段值引入重复项：

select distinct pt.assignment_id,
  coalesce(st.effective_start_date, pt.effective_start_date) as effective_start_date,
  coalesce(st.effective_end_date, pt.effective_end_date) as effective_end_date,
  coalesce(st.action_reason, pt.action_reason) as action_reason
from prod_table pt
left join staging_table st
on st.assignment_id = pt.assignment_id
and (st.effective_start_date = pt.effective_start_date
  or st.effective_end_date = pt.effective_end_date)
order by assignment_id, effective_start_date

如果你想更新主表中的数据，那么你可以使用merge，使用基本相同的逻辑，并使用delete where子句来删除现在重复的数据：

merge into prod_table pt
using staging_table st
on (
  st.assignment_id = pt.assignment_id
  and (st.effective_start_date = pt.effective_start_date
    or st.effective_end_date = pt.effective_end_date)
)
when matched then update set
  pt.effective_end_date = st.effective_end_date,
  pt.action_reason = st.action_reason
delete where pt.effective_start_date != st.effective_start_date

它更新所有匹配的行，但只更新结束日期和原因，而保留原始的开始日期;然后删除那些与暂存开始日期不匹配的匹配行，只留下一行。

select * from prod_table

fiddle
但是如果你有与现有日期不完全匹配的分期数据，那么它可能会变得更加复杂;如果是这样的话，那么你需要问一个新的问题，清楚地列出你需要处理的所有场景（间隙，重叠，冲突，...）以及你对每一个场景的预期行为。

虽然我不太确定你到底想得到什么（条件是什么），但在我看来，这似乎可以用MODEL clause来完成，它允许你像用excel工作表一样管理数据。
下面的代码并没有完成这项工作，但向您展示了一些调节和操作数据的方法：
样本数据：

--  stg table
        ID EFF_START EFF_END   REASON            
---------- --------- --------- ------------------
     30001 06-MAY-21 31-DEC-12 Manager Change    -- end year 4712
     
-- prod table
        ID EFF_START EFF_END   REASON                                                                                             
---------- --------- --------- ------------------
     30001 01-JAN-10 02-FEB-15 Name Change       
     30001 03-FEB-15 05-MAY-21 Name Change       
     30001 06-MAY-21 03-MAR-23 Name Change       
     30001 04-MAR-23 31-DEC-12 Name Change      -- end year 4712

主SQL：

Select   ID, S_EFF_START, S_EFF_END, S_REASON, RN, EFF_START, EFF_END, DATSTAT, REASON, RSNSTAT
From
    ( Select  ROW_NUMBER() OVER(Partition By p.ID Order By p.ID, p.EFF_START) "RN",
              p.ID "ID", p.EFF_START "EFF_START", p.EFF_END "EFF_END", p.REASON "REASON",
              s.EFF_START "S_EFF_START", s.EFF_END "S_EFF_END", s.REASON "S_REASON"
      From stg s
      Left Join prod p ON( 1 = 1 ) 
    )
    MODEL Partition By (ID)
          Dimension By (RN, S_EFF_START, S_EFF_END)
          Measures (EFF_START, EFF_END, REASON, S_REASON, Cast('' As VarChar2(64)) "DATSTAT", Cast('' As VarChar2(64)) "RSNSTAT")
    Rules (
              EFF_START[ANY, ANY, ANY] =  CASE  WHEN  EFF_START[CV(), CV(), CV()] Between CV(S_EFF_START) And CV(S_EFF_END) 
                                                THEN  CV(S_EFF_START) 
                                          ELSE EFF_START[CV(), CV(), CV()]
                                          END,
              EFF_END[ANY, ANY, ANY]   =  CASE  WHEN  EFF_END[CV(), CV(), CV()] Between CV(S_EFF_START) And CV(S_EFF_END) 
                                                THEN  CV(S_EFF_END) 
                                          ELSE  EFF_END[CV(), CV(), CV()]
                                          END,
              REASON[ANY, ANY, ANY]    =  CASE  WHEN  EFF_START[CV(), CV(), CV()] Between CV(S_EFF_START) And CV(S_EFF_END) And 
                                                      EFF_END[CV(), CV(), CV()] Between CV(S_EFF_START) And CV(S_EFF_END) 
                                                THEN  S_REASON[CV(), CV(), CV()] 
                                          ELSE  REASON[CV(), CV(), CV()]
                                          END,
              --
              DATSTAT[ANY, ANY, ANY]   =  CASE  WHEN   EFF_START[CV(), CV(), CV()] Between CV(S_EFF_START) And CV(S_EFF_END) OR
                                                      EFF_END[CV(), CV(), CV()] Between CV(S_EFF_START) And CV(S_EFF_END) 
                                                THEN  'Date(s) changed to one(s) from stg table' 
                                          ELSE 'No change'
                                          END,
              RSNSTAT[ANY, ANY, ANY]  =  CASE  WHEN   EFF_START[CV(), CV(), CV()] Between CV(S_EFF_START) And CV(S_EFF_END) OR
                                                      EFF_END[CV(), CV(), CV()] Between CV(S_EFF_START) And CV(S_EFF_END) 
                                                THEN  'REASON changed' 
                                          ELSE 'No change'
                                          END
          )

结果：

--  
--      R e s u l t : 
        ID S_EFF_START S_EFF_END S_REASON                       RN EFF_START EFF_END   DATSTAT                                     REASON                 RSNSTAT                
---------- ----------- --------- ---------------------- ---------- --------- --------- ------------------------------------------- ---------------------- -----------------------
     30001 06-MAY-21   31-DEC-12 Manager Change                  1 01-JAN-10 02-FEB-15 No change                                   Name Change            No change              
     30001 06-MAY-21   31-DEC-12 Manager Change                  2 03-FEB-15 05-MAY-21 No change                                   Name Change            No change              
     30001 06-MAY-21   31-DEC-12 Manager Change                  3 06-MAY-21 31-DEC-12 Date(s) changed to one(s) from stg table    Manager Change         REASON changed         
     30001 06-MAY-21   31-DEC-12 Manager Change                  4 06-MAY-21 31-DEC-12 Date(s) changed to one(s) from stg table    Manager Change         REASON changed

它看起来有点复杂，但它是可靠和快速的。如果你能看一下文档-它迟早会派上用场的。