Question

给出以下数据库结构（它只是一个演示结构）：

order:
| id | orderedat |

order_item:
| id | id_order | id_article | price | currency |

我想得到的是每篇文章的最新订单价格：

| id_article | orderedat | price | currency |

我写这篇陈述是为了实现这个目标：

WITH "last_ordered" AS (   
  SELECT "oi"."id_article", "oi"."id" as "id_oi", MAX("o"."orderedat")   
  FROM "order" "o", "order_item" "oi"   
  WHERE ("oi"."id_order" = "o"."id")            
  GROUP BY "oi"."id", "oi"."id_id_article"    
)   
SELECT "oi"."id_article", "o"."orderedat", "oi"."price", "oi"."currency"   
FROM "order_item" "oi"   
INNER JOIN "last_ordered" ON ("oi"."id" = "last_ordered"."id_oi")   
INNER JOIN "order" "o" ON ("oi"."id_order" = "o"."id")

但是，group by是错误的，因为它为文章链接的每个order_item返回一行。另一方面，专栏＆＃34; oi.id＆＃34;必须出现在GROUP BY子句中。我不知道如何解决这个问题。有人能帮帮我吗？

也许这个问题有另一个解决方案。

提前致谢：）

Answer 1

WITH "last_ordered" AS (   
  SELECT "o".*,"oi".*,
         ROW_NUMBER() OVER (PARTITION BY "oi"."id_id_article" ORDER BY "o"."orderedat" DESC) as Rnum 
  FROM "order" "o"
  INNER JOIN "order_item" "oi"   
  ON "oi"."id_order" = "o"."id"            

) 

SELECT * FROM  last_ordered
WHERE Rnum =1

Answer 2

select distinct on (id_article)
    id_article, orderedat, price, currency
from
    order o
    inner join
    order_item oi on o.id = oi.id_order
order by id_article, orderedat desc

检查distinct on：

SELECT DISTINCT ON（expression [，...]）仅保留给定表达式求值的每组行的第一行。使用与ORDER BY相同的规则解释DISTINCT ON表达式（参见上文）。注意＆＃34;第一行＆＃34;除非使用ORDER BY来确保首先出现所需的行，否则每个集合都是不可预测的。

如何使用其他列选择最大日期

2 个答案: