具有不同 ORDER BY 的 PostgreSQL DISTINCT ON [英] PostgreSQL DISTINCT ON with different ORDER BY
问题描述
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
但我收到此错误:
PG::Error: ERROR: SELECT DISTINCT ON 表达式必须匹配初始 ORDER BY 表达式
PG::Error: ERROR: SELECT DISTINCT ON expressions must match initial ORDER BY expressions
添加 address_id
作为第一个 ORDER BY
表达式会消除错误,但我真的不想在 address_id
上添加排序.是否可以不按 address_id
排序?
Adding address_id
as first ORDER BY
expression silences the error, but I really don't want to add sorting over address_id
. Is it possible to do without ordering by address_id
?
推荐答案
文档说:
DISTINCT ON ( expression [, ...] ) 仅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 表达式必须匹配最左边的 ORDER BY 表达式.
DISTINCT ON ( expression [, ...] ) keeps only the first row of each set of rows where the given expressions evaluate to equal. [...] Note that the "first row" of each set is unpredictable unless ORDER BY is used to ensure that the desired row appears first. [...] The DISTINCT ON expression(s) must match the leftmost ORDER BY expression(s).
因此您必须将 address_id
添加到订单中.
So you'll have to add the address_id
to the order by.
或者,如果您要查找包含每个 address_id
最近购买的产品的完整行,并且该结果按 purchased_at
排序,那么您正在尝试解决每组最大 N 的问题,可以通过以下方法解决:
Alternatively, if you're looking for the full row that contains the most recent purchased product for each address_id
and that result sorted by purchased_at
then you're trying to solve a greatest N per group problem which can be solved by the following approaches:
适用于大多数 DBMS 的通用解决方案:
The general solution that should work in most DBMSs:
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
基于@hkf 的回答的更面向 PostgreSQL 的解决方案:
A more PostgreSQL-oriented solution based on @hkf's answer:
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:选择按某列排序并在另一列上不同的行
Problem clarified, extended and solved here: Selecting rows ordered by some column and distinct on another
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