Postgresql获取具有多列的每个组的最大值[重复]

Question

我有下表

CREATE table test
(
    random_number INT,
    band VARCHAR(255),
    member_name VARCHAR(255)
);

INSERT INTO test VALUES(2300,'Metallica', 'Kirk');
INSERT INTO test VALUES(2000,'Metallica', 'Lars');
INSERT INTO test VALUES(2500,'Metallica', 'James');
INSERT INTO test VALUES(2800,'Metallica', 'Roberto');
INSERT INTO test VALUES(100,'SkidRow', 'Sebastian');
INSERT INTO test VALUES(140,'SkidRow', 'Rachel');
INSERT INTO test VALUES(110,'SkidRow', 'Scott');
INSERT INTO test VALUES(150,'SkidRow', 'Dave');
INSERT INTO test VALUES(100,'SkidRow', 'Rob');
INSERT INTO test VALUES(500,'Motorhead', 'Lemmy');
INSERT INTO test VALUES(100,'Motorhead', 'Mikkey');
INSERT INTO test VALUES(200,'Motorhead', 'Phil');

我怎样才能获得每个乐队中最大的random_number 并返回如下内容：

random_number |   band    | member_name
-----------------------------------------
     2800     | Metallica |  Roberto
     150      | SkidRow   |  Dave
     500      | Motorhead |  Lemmy

Answer 1

使用distinct on:

select distinct on (band) t.*
from test t
order by band, random_number desc;

Here 是一个 dbfiddle。

distinct on 是一个非常方便的 Postgres 扩展。对于大型数据集的性能，您需要(band, random_number desc) 上的索引。

Answer 2

通过分组和加入表找到最大的random_number：

select t.* 
from test t inner join (
  select band, max(random_number) maxnum from test group by band
) g
on g.band = t.band and g.maxnum = t.random_number

见demo

Answer 3

使用MAX()

SELECT max(random_number), band FROM test GROUP BY band

见：https://rextester.com/BEZD44968

结果是：

no  max band
1   2800 Metallica
2   500 Motorhead
3   150 SkidRow