PostgreSQL GROUP BY：另一个MAX的SELECT列，第三列= x [英] PostgreSQL GROUP BY: SELECT column on MAX of another WHERE a third column = x

问题描述

假设我们在PostgreSQL中有两个表：

Let's suppose we have two tables in PostgreSQL:

表 citizens

Table "citizens"

country_ref   citizen_name    entry_date
-----------------------------------------------------
0             peter           2013-01-14 21:00:00.000
1             fernando        2013-01-14 20:00:00.000
0             robert          2013-01-14 19:00:00.000
3             albert          2013-01-14 18:00:00.000
2             esther          2013-01-14 17:00:00.000
1             juan            2013-01-14 16:00:00.000
3             egbert          2013-01-14 15:00:00.000
1             francisco       2013-01-14 14:00:00.000
3             adolph          2013-01-14 13:00:00.000
2             emilie          2013-01-14 12:00:00.000
2             jacques         2013-01-14 11:00:00.000
0             david           2013-01-14 10:00:00.000

表国家

country_id     country_name   country_group
-------------------------------------------
0              england        0
1              spain          0 
2              france         1
3              germany        1

现在，我想获取给定country_group中每个国家/地区在公民表上最后输入的公民。

Now I want to obtain the last entered citizen on the "citizens" table for each country of a given country_group.

到目前为止，我最好的尝试是这个查询（我们称它为Query_1）：

My best try so far is this query (Let's call it Query_1) :

SELECT country_ref, MAX(entry_date) FROM citizens 
LEFT JOIN countries ON country_id = country_ref 
WHERE country_group = 1 GROUP BY country_ref

输出：

country_ref   max
---------------------------------
3             2013-01-14 18:00:00
2             2013-01-14 17:00:00

所以我可以这样做：

SELECT citizen_name FROM citizens WHERE (country_ref, entry_date) IN (Query_1)

...这将为我提供所需的输出： albert 和 esther 。

... which will give me the output I'm looking for: albert and esther.

但是我更愿意实现单个查询中。我想知道是否有可能？

But I'd prefer to achieve this in a single query. I wonder if it's possible?

推荐答案

这应该是最简单，最快的：

This should be simplest and fastest:

SELECT DISTINCT ON (i.country_ref)
       i.citizen_name
FROM   citizens  i
JOIN   countries o ON o.country_id = i.country_ref
WHERE  o.country_group = 1
ORDER  BY i.country_ref, i.entry_date DESC

只需将它们添加到 SELECT 列表中，就可以轻松地从两个表中返回更多列。

SQL小提琴。

You can easily return more columns from both tables by simply adding them to the SELECT list.
SQL Fiddle.

详细信息，此相关答案中的链接和说明：

Details, links and explanation in this related answer:

• 在每个GROUP BY组中选择第一行？

• Select first row in each GROUP BY group?

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