如何使用 max() 函数使用 SQL / postgreSQL 检索最近的行

Question

我有一个看起来像这样的数据集 example data set.

一个域下有多个用户。每个 email_domain 我只想要一行，并且该行应该对应于 max(last_login) 值。简而言之，我只希望来自同一域的所有用户中最后登录的 email_domain 用户。

我试过一个看起来像这样的查询

select *
FROM
(
select LOWER(SUBSTRING(ua.email FROM POSITION ('@' IN ua.email) + 1)) AS email_domain, last_login, last_name, first_name, email, phone  
from user_with_address ua 
order by email_domain
) as A
group by email_domain, last_login, last_name, first_name, email, phone  
having last_login = max(last_login)
order by email_domain

我仍然得到一个包含每个电子邮件域的多个值的列表，我做错了什么？请帮忙。

免责声明：我对 SQL 有基础->中级知识。

Answer 1

一种选择是使用ROW_NUMBER()并为每组相同的电子邮件域记录保留最近的登录记录。

SELECT t.email_domain, t.last_login, t.last_name, t.first_name, t.email, t.phone
FROM
(
    SELECT a.*,
           ROW_NUMBER() OVER (PARTITION BY a.email_domain ORDER BY a.last_login DESC) rn
    FROM
    (
        SELECT LOWER(SUBSTRING(ua.email FROM POSITION ('@' IN ua.email) + 1)) AS email_domain,
               last_login, last_name, first_name, email, phone
        FROM user_with_address ua 
    ) a
) t
WHERE t.rn = 1
ORDER BY t.email_domain

请注意，我实际上在这里进行了两次子查询，以避免重复计算电子邮件域的代码。如果不是这样，我们可以只用一个子查询来完成这个。我们可以在这里使用单个子查询，但是查询会有点难读。

Answer 2

我喜欢 Tim Biegeleisen 的回答，但这更简单，SQL 明智。虽然不知道性能差异。

select 
  LOWER(SUBSTRING(ua.email FROM POSITION ('@' IN ua.email) + 1)) AS email_domain, 
  last_login, 
  last_name, 
  first_name, 
  email, 
  phone  
from user_with_address ua 
where last_login = (select max(last_login) 
                    from user_with_address ua2
                    where LOWER(SUBSTRING(ua.email FROM POSITION ('@' IN ua.email) =
                          LOWER(SUBSTRING(ua2.email FROM POSITION ('@' IN ua2.email))
order by email_domain;

Answer 3

使用distinct on ()

select distinct on (email_domain) *
FROM (
  select lower(split_part(email, '@', 2)) AS email_domain, 
         last_login, 
         last_name, 
         first_name, 
         email, 
         phone  
  from user_with_address
) as A
order by email_domain, last_login desc;

我还采纳了 Patrick 的建议来简化从电子邮件中提取域的表达式。

如何使用 max() 函数使用 SQL / postgreSQL 检索最近的行

How to use max() function to retrieve the most recent row using SQL / postgreSQL

sql

postgresql

greatest-n-per-group