MODE聚集功能

Question

我有以下表格：

客戶

customer_id name 
---------------- 
1   bob 
2   alice 
3   tim 

購買

id customer_id item_bought 
-------------------------- 
1 1   hat 
2 1   shoes 
3 2   glasses 
3 2   glasses 
4 2   book 
5 3   shoes 
6 1   hat 

我想下面的結果：

customer_name item_bought_most_often 
------------------------------------ 
bob   hat 
alice   glasses 
tim   shoes 

我會做這樣的（實際上沒有試過，只是想法）：

SELECT customer.name as customer_name, 
    MODE(item_bought), as item_bought_most_ofen 
FROM customers 
INNER JOIN purchases USING (customer_id) 
GROUP_BY customer_id 

然而，MODE aggregation function不存在紅移。

看來Redshift user defined functions只是常規的標量函數，而不是聚合函數。所以我不認爲我可以自己定義它。

任何解決方法？

Answer 1

您可以通過使用row_number()模仿mode()：

select name, item_bought 
from (select c.name, p.item_bought, count(*) as cnt, 
      row_number() over (order by count(*) desc) as seqnum 
     from customers c join 
      purchases p 
      using (customer_id) 
     group by c.name, p.item_bought 
    ) cp 
where seqnum = 1; 

Answer 2

你可以先COUNT每個人購買，然後用RANK()窗口函數：

SELECT name AS customer_name, item_bought AS item_bought_most_often 
FROM(SELECT name,item_bought,RANK() OVER(PARTITION BY name ORDER BY cnt DESC) rnk 
    FROM (SELECT c.name, p.item_bought, COUNT(*) AS cnt 
      FROM customers c 
      JOIN purchases p 
      ON p.customer_id = c.customer_id 
      GROUP BY c.name, p.item_bought) AS s1) AS s2 
WHERE rnk = 1; 

LiveDemo

輸出：

╔═══════════════╦════════════════════════╗ 
║ customer_name ║ item_bought_most_often ║ 
╠═══════════════╬════════════════════════╣ 
║ alice   ║ glasses    ║ 
║ bob   ║ hat     ║ 
║ tim   ║ shoes     ║ 
║ zoe   ║ pencil     ║ 
║ zoe   ║ book     ║ 
╚═══════════════╩════════════════════════╝ 

注：

RANK將處理多個最常用的值。