Oracle爲列表中的每個項目選擇一個記錄樣本

Question

假設我有一個包含列id，顏色，項目的表格產品。

select distinct color from product 

有沒有一種方法沒有做一堆的聯盟，從產品表中爲每種顏色選擇最多2條記錄？

我很想能夠做到像

select distinct color from product sample 2 

Answer 1

不要使用union小號！只需使用row_number()：

select p.* 
from (select p.*, row_number() over (partition by color order by color) as seqnum 
     from product p 
    ) p 
where seqnum <= 2; 

Answer 2

select id, color, item 
from (select id, color, item, 
       row_number() over (partition by color order by null) as rn 
     from product 
     ) 
where rn <= 2;