得到N行

Question

假設我有一個表如之間的距離：

ID,visit, Concentration 
1, 1, 4.0 
2, 2, 2.0 
3, 3, 1.5 
4, 4, 5.0 
5, 1, 7.0 
6, 2, 1.0 
7, 3, 5.0 
8, 4, 12.0 

該表具有的4個濃度2個亞組（該分區按id製成，訪問）。在每個組中，我想遍歷所有4個濃度並返回兩個id，根據濃度訪問最近的一個。

所以查詢將返回一個結果，像這樣的：

ID, visit, concentration 
2, 2, 2.0 
3, 3, 1.5 
5, 1, 7.0 
7, 3, 5.0 

感謝

Answer 1

您可以分配組以不同的方式。假設所有大小都相同，則可以按照ID的順序枚舉訪問次數。然後，我認爲這是最簡單的返回每行一對：

with g as (
     select t.*, row_number() over (partition by visit order by id) as grp 
     from t 
    ) 
select * 
from (select g1.id as id1, g1.visit as visit1, g1.concentration as concentration1, 
      g2.id as id2, g2.visit as visit2, g2.concentration as concentration2 
      row_number() over (partition by g1.grp 
           order by abs(g1.concentration - g2.concentration) 
           ) as seqnum 
     from g g1 join 
      g g2 
      on g1.grp = g2.grp and g1.id < g2.id 
    ) g 
where seqnum = 1;