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:
SELECT DISTINCT ON表达式必须匹配表达式的初始顺序
添加address_id作为第一个ORDER BY表达式可以消除错误，但我真的不想在address_id上添加排序。有没有可能不按address_id排序?
当前回答
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，因此不能在同一个SELECT中按不同的列排序。
如果你想从每个集合中选择一个特定的行，只在子查询中使用一个额外的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，请使用DESC NULLS LAST -并匹配您的索引以获得最佳性能。看到的:
按列ASC排序，但空值首先? 为什么ORDER BY NULLS LAST会影响主键上的查询计划?
相关，有更多解释:
在每个GROUP BY GROUP中选择第一行? 按列ASC排序，但空值首先?
其他回答
你也可以通过使用group by子句来做到这一点
SELECT purchases.address_id, purchases.* FROM "purchases"
WHERE "purchases"."product_id" = 1 GROUP BY address_id,
purchases.purchased_at ORDER purchases.purchased_at DESC
它也可以使用以下查询和其他答案来解决。
WITH purchase_data AS (
SELECT address_id, purchased_at, product_id,
row_number() OVER (PARTITION BY address_id ORDER BY purchased_at DESC) AS row_number
FROM purchases
WHERE product_id = 1)
SELECT address_id, purchased_at, product_id
FROM purchase_data where row_number = 1
对于任何使用Flask-SQLAlchemy的人来说，这对我来说都是有效的
from app import db
from app.models import Purchases
from sqlalchemy.orm import aliased
from sqlalchemy import desc
stmt = Purchases.query.distinct(Purchases.address_id).subquery('purchases')
alias = aliased(Purchases, stmt)
distinct = db.session.query(alias)
distinct.order_by(desc(alias.purchased_at))
Window函数可以一次性解决这个问题:
SELECT DISTINCT ON (address_id)
LAST_VALUE(purchases.address_id) OVER wnd AS address_id
FROM "purchases"
WHERE "purchases"."product_id" = 1
WINDOW wnd AS (
PARTITION BY address_id ORDER BY purchases.purchased_at DESC
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)
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，因此不能在同一个SELECT中按不同的列排序。
如果你想从每个集合中选择一个特定的行，只在子查询中使用一个额外的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，请使用DESC NULLS LAST -并匹配您的索引以获得最佳性能。看到的:
按列ASC排序，但空值首先? 为什么ORDER BY NULLS LAST会影响主键上的查询计划?
相关，有更多解释:
在每个GROUP BY GROUP中选择第一行? 按列ASC排序，但空值首先?
