你可以用 LAG() 窗口功能：

select 
  nullif(Month, lag(Month) over (order by null)) Month,
  Product
from tablename

请看演示。
结果：

> MONTH | PRODUCT                       
> :---- | :-----------------------------
> March | ENVOY & External Keyboard (22)
>       | ENVOY & External Keyboard (22)
>       | ENVOY & External Keyboard (22)
>       | ENVOY & External Keyboard (22)
>       | ENVOY & External Keyboard (22)
>       | ENVOY & External Keyboard (22)
> April | ENVOY & External Keyboard (22)
>       | ENVOY & External Keyboard (22)
>       | ENVOY & External Keyboard (22)
>       | ENVOY & External Keyboard (22)
>       | ENVOY & External Keyboard (22)

小菜一碟sql*plus：

SQL> break on month
SQL>
SQL> select month, product from test order by month desc;

MONTH      PRODUCT
---------- ------------------------------
March      ENVOY & External Keyboard (22)
           ENVOY & External Keyboard (22)
           ENVOY & External Keyboard (22)
           ENVOY & External Keyboard (22)
           ENVOY & External Keyboard (22)
           ENVOY & External Keyboard (22)
April      ENVOY & External Keyboard (22)
           ENVOY & External Keyboard (22)
           ENVOY & External Keyboard (22)
           ENVOY & External Keyboard (22)
           ENVOY & External Keyboard (22)

11 rows selected.

SQL>

这种类型的操作最好在应用程序级别完成。你可以用 row_number() ，但您需要小心：

select (case when seqnum = 1 then month end) as month, product
from (select t.*,
             row_number() over (partition by month order by month) as seqnum
      from t
     ) t
order by month, seqnum;

请注意，sql查询以不确定的顺序返回结果。所以你需要一个外罩 order by 以保证排序（即“第一”行具有值）。因为您要处理的是完全重复的，所以没有唯一的排序键，所以即使对 row_number() 可能产生不同的结果——因此使用子查询。