如何在引用 table 的列中查询具有最大值的行？

Question

有一些流行的情况，需要从 table 中 select 具有最大值（或最小值或其他聚合函数结果）的行。我的情况更复杂 - 它在图片中添加了 JOIN 。

这是一个例子：

CREATE TABLE spacecraft (
  id   serial PRIMARY KEY, 
  name text NOT NULL
);

CREATE TABLE launch (
  id            serial PRIMARY KEY,
  spacecraft_id int REFERENCES spacecraft(id) NOT NULL,
  started       timestamptz NOT NULL,
  success       bool NOT NULL
);

INSERT INTO spacecraft (id, name) VALUES 
    (1, 'Bebop'), 
    (2, 'Serenity'), 
    (3, 'Death Start');

INSERT INTO launch (spacecraft_id, started, success) VALUES 
    (1, 'January 8 04:05:06 2999 UTC', true), 
    (2, 'December 1 01:00:00 3432 UTC', true), 
    (3, 'February 15 00:00:00 4521 UTC', false), 
    (3, 'July 10 12:05:00 4525 UTC', true);

http://sqlfiddle.com/#!15/a1921

如何select每个航天器的最后发射结果（launch.success）？

更新 #1

这是我目前的解决方案：

SELECT DISTINCT S.*, last_launch, L2.success FROM spacecraft AS S
LEFT OUTER JOIN (SELECT *, MAX(started) OVER (PARTITION BY spacecraft_id) AS last_launch FROM launch) AS L1
ON L1.spacecraft_id=S.id
LEFT OUTER JOIN launch AS L2
ON L2.started=L1.last_launch;

http://sqlfiddle.com/#!15/45618/38

Answer 1

如果你只想获得最大的成功，那么你已经添加了一个条件来过滤只有成功。并像您一样使用 Window Functions：

select spacecraft.* , max(started) over (partition by spacecraft.id)  
from spacecraft 
    left outer join 
    launch 
    on spacecraft.id = spacecraft_id
where success

SQL FIDDLE

Answer 2

这是我的解决方案。

SELECT s.id, s.name, l.last_started
FROM spacecraft AS s
INNER JOIN (SELECT spacecraft_id, MAX(started) AS last_started FROM launch WHERE success GROUP BY spacecraft_id) AS l
ON s.id = l.spacecraft_id

http://sqlfiddle.com/#!15/45618/50

Answer 3

假设您想要结果中 spacecraft 的所有列，加上最新的 launch 和同一行的 success。

每个航天器的几次发射速度更快：

SELECT s.*, l.last_launch, l.success
FROM   spacecraft s
LEFT   JOIN (
   SELECT DISTINCT ON (spacecraft_id)
          spacecraft_id, started AS last_launch, success
   FROM   launch
   ORDER  BY spacecraft_id, started DESC
   ) l ON l.spacecraft_id = s.id;

每个航天器的多次发射速度更快：

SELECT s.*, l.last_launch, l.success
FROM   spacecraft s
LEFT   JOIN LATERAL (
   SELECT started AS last_launch, success
   FROM   launch
   WHERE  spacecraft_id = s.id
   ORDER  BY started DESC
   LIMIT  1
   ) l ON true;

LATERAL 需要 Postgres 9.3+。
LEFT JOIN 包括没有发射的航天器。否则使用 JOIN.

快速中任一查询的基本要素是 (spacecraft_id, started) 上的多列索引。对于这个特定的查询，success 到索引以允许仅索引扫描将是有益的：

CREATE INDEX launch_foo_idx ON launch (spacecraft_id, started DESC, success);

SQL Fiddle.

详细解释：

Optimize GROUP BY query to retrieve latest record per user
Select first row in each GROUP BY group?

如何在引用 table 的列中查询具有最大值的行？

How to query row with max value in a column in a referencing table?

sql

postgresql

join

greatest-n-per-group

更新 #1