如何在Postgresql的一行中获取第一个和最后一个值[关闭]

Posted 2023-03-31

【中文标题】如何在Postgresql的一行中获取第一个和最后一个值[关闭]

【英文标题】：How to get first and last value on one row in Postgresql [closed]

【发布时间】：2020-09-24 23:58:46

【问题描述】：

输入：

postgres=> select player_1, row_number() over (order by id) as r1, row_number() over (order by id desc) as r2 from (select id, player_1 from matches order by id limit 5) as foo;

      player_1      | r1 | r2 
--------------------+----+----
 Kafelnikov Yevgeny |  1 |  5
 Dreekmann Hendrik  |  2 |  4
 Courier Jim        |  3 |  3
 Krajicek Richard   |  4 |  2
 Rafter Patrick     |  5 |  1

(5 rows)

预期：

      first      | last
--------------------+----+----
 Kafelnikov Yevgeny |  Rafter Patrick 

我尝试了以下方法，但它有 5 行，我需要它们都放在一行上，就像上面一样。

postgres=> select case when r1 = 1 then player_1 end, case when r2 = 1 then player_1 end from (select player_1, row_number() over (order by id) as r1, row_number() over (order by id desc) as r2 from (select id, player_1 from matches order by id limit 5) as foo) as too;
        case        |      case      
--------------------+----------------
 Kafelnikov Yevgeny | 
                    | 
                    | 
                    | 
                    | Rafter Patrick
(5 rows)

【参考方案1】：

你在正确的轨道上。您可以使用这些行号进行条件聚合：

select 
    max(player_1) filter(where r1 = 1) first_player,
    max(player_1) filter(where r2 = 1) last_player
from (
    select player_1, 
        row_number() over (order by id) as r1, 
        row_number() over (order by id desc) as r2 
    from (select * from matches order by id limit 5) t
    order by id limit 5
) t;

请注意，您也可以使用 first_value() 和 distinct 执行此操作：

select distinct
    first_value(player_1) over(order by id) first_player,
    first_value(player_1) over(order by id desc) last_player
from (select * from matches order by id limit 5) t

Demo on DB Fiddle

【参考方案2】：

另一种方法如下 -

select 
    max(case when r1 = 1 then player_1 end) as first_player,
    max(case when r2 = 1 then player_1 end) as last_player
from (
      select player_1, 
             row_number() over (order by id) as r1, 
             row_number() over (order by id desc) as r2 
        from (select * from mytable order by id limit 3) t
       order by id limit 5
     ) t;