Question

假设您在PostgreSQL上有以下users表：

id | group_id |    name | age
---|----------|---------|----
 1 |        1 |    adam |  10
 2 |        1 |     ben |  11
 3 |        1 | charlie |  12   <-
 3 |        2 |  donnie |  20  
 4 |        2 |    ewan |  21   <-
 5 |        3 |    fred |  30   <-

如何仅按group_id（用箭头标记的）从最早的用户查询所有列？

我尝试过使用group by，但是一直按"users.id" must appear in the GROUP BY clause。

（注意：我必须将查询工作到Rails AR模型范围中。）

Answer 1

您可以使用ROW_NUMBER/RANK（如果可能的话）窗口函数：

SELECT * 
FROM (SELECT *,ROW_NUMBER() OVER(PARTITION BY group_id ORDER BY age DESC) AS rn
      FROM tab) s
WHERE s.rn = 1;

Answer 2

您可以在联接中使用子查询并增加结果

select m.* 
from  users m
inner join (
  select  group_id, max(age) max_age
  from users 
  group by group_id
) AS t on (t.group_id = m.group_id and t.max_age  = m.age)

Answer 3

经过一些挖掘，您可以使用PostgreSQL的select distinct on (users.group_id) users.* from users order by users.group_id, users.age desc; -- you might want to add extra column in ordering in case 2 users have the same age for same group_id：

User
  .select('DISTINCT ON (users.group_id), users.*')
  .order('users.group_id, users.age DESC')

在Rails中翻译为：

DISTINCT ON

有关.InitialFileName = CurrentProject.Path 'Or any path的一些文档：https://www.postgresql.org/docs/9.3/sql-select.html#SQL-DISTINCT

工作示例：https://www.db-fiddle.com/f/t4jeW4Sy91oxEfjMKYJpB1/0

仅查询组中具有最大值的记录

3 个答案: