具有不同ORDER BY的PostgreSQL DISTINCT ON
我想运行这个查询：
SELECT DISTINCT ON (address_id) purchases.address_id, purchases.*
FROM purchases
WHERE purchases.product_id = 1
ORDER BY purchases.purchased_at DESC但是我得到了这个错误：
PG::Error: ERROR: SELECT DISTINCT ON表达式必须与初始ORDER BY表达式匹配
添加address_id作为第一个ORDER BY表达式可以消除这个错误，但我真的不想在address_id上添加排序。是否可以不按address_id排序
回答 7
Stack Overflow用户
发布于 2012-03-21 06:08:32
文档中写道：
表达式DISTINCT ON (
，... )只保留给定表达式求值为相等的每组行中的第一行。..。请注意，除非使用ORDER BY来确保所需的行先出现，否则每个集合的“第一行”是不可预测的。..。DISTINCT ON表达式必须与最左侧的ORDER BY表达式匹配。
因此，您必须将address_id添加到order by。
或者，如果您正在查找包含每个address_id的最新购买产品的整行，并且该结果按purchased_at排序，那么您将尝试解决每组最大N的问题，该问题可以通过以下方法解决：
应该在大多数DBMS中工作的一般解决方案：
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一个更加面向PostgreSQL的解决方案，基于@hkf的回答：
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问题在这里得到澄清、扩展和解决：Selecting rows ordered by some column and distinct on another
Stack Overflow用户
发布于 2012-03-21 06:04:34
可以在子查询中按address_id排序，然后在外部查询中按所需排序。
SELECT * FROM
(SELECT DISTINCT ON (address_id) purchases.address_id, purchases.*
FROM "purchases"
WHERE "purchases"."product_id" = 1 ORDER BY address_id DESC )
ORDER BY purchased_at DESCStack Overflow用户
发布于 2012-03-21 06:40:27
子查询可以解决这个问题：
SELECT *
FROM (
SELECT DISTINCT ON (address_id) *
FROM purchases
WHERE product_id = 1
) p
ORDER BY purchased_at DESC;ORDER BY中的前导表达式必须与DISTINCT ON中的列一致，因此不能按同一SELECT中的不同列排序。
如果要从每个集合中选取特定行，则仅在子查询中使用额外的ORDER BY：
SELECT *
FROM (
SELECT DISTINCT ON (address_id) *
FROM purchases
WHERE product_id = 1
ORDER BY address_id, purchased_at DESC -- get "latest" row per address_id
) p
ORDER BY purchased_at DESC;如果purchased_at可以为NULL，则使用DESC NULLS LAST -并匹配您的索引以获得最佳性能。请参见：
- Sort by column ASC, but NULL values first?
- Why does ORDER BY NULLS LAST affect the query plan on a primary key?
相关的，有更多解释：
https://stackoverflow.com/questions/9795660
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