具有不同 ORDER BY 的 PostgreSQL DISTINCT ON

发布于03月21日

我想运行以下查询:

SELECT DISTINCT ON (address_id) purchases.address_id, purchases.*
FROM purchases
WHERE purchases.product_id = 1
ORDER BY purchases.purchased_at DESC

但我有一个错误:

PG::Error:Error:SELECT DISTINCT ON表达式必须按表达式匹配初始顺序

添加address_id作为第一个ORDER BY表达式可以消除错误，但我真的不想添加排序超过address_id.有可能不在address_id点之前订购吗？

推荐答案

文件显示:

不同于(表达式[，…])的只保留给定表达式计算结果相等的每组行的第一行.[...] 请注意，每个集合的"第一行"是不可预测的，除非使用ORDER BY来确保所需的行首先出现.[...] DISTINCT ON表达式必须与最左边的ORDER BY表达式匹配.

Official documentation

所以你必须在订单上加上address_id.

或者，如果你正在寻找每address_id个包含最新购买产品的整行，并且结果按purchased_at排序，那么你正在try 解决一个最大的N个每组问题，该问题可以通过以下方法解决:

适用于大多数DBMS的通用解决方案:

SELECT t1.* FROM purchases t1
JOIN (
    SELECT address_id, max(purchased_at) max_purchased_at
    FROM purchases
    WHERE product_id = 1
    GROUP BY address_id
) t2
ON t1.address_id = t2.address_id AND t1.purchased_at = t2.max_purchased_at
ORDER BY t1.purchased_at DESC

一个基于@hkf答案的更面向PostgreSQL的解决方案:

SELECT * FROM (
  SELECT DISTINCT ON (address_id) *
  FROM purchases 
  WHERE product_id = 1
  ORDER BY address_id, purchased_at DESC
) t
ORDER BY purchased_at DESC

问题在此得到澄清、扩展和解决:Selecting rows ordered by some column and distinct on another