Question

有一个由~10列组成的表，如何选择时间戳是最新列的那些列的所有组合？

Country City    Street  ... Timestamp
Poland  Warsaw  OldSt   ... 5/25/2018
Poland  Warsaw  NewSt   ... 6/14/2018
Germany Berlin  OldSt   ... 5/30/2018
Germany Berlin  NewSt   ... 6/14/2018
France  Paris   SomeSt  ... 6/14/2018

会得到我

Country City    Street  ... Timestamp
Poland  Warsaw  NewSt   ... 6/14/2018
Germany Berlin  NewSt   ... 6/14/2018
France  Paris   SomeSt  ... 6/14/2018

我相信有很多方法可以解决这个问题。但是效果最好的是什么？

表格的DDL：

-- auto-generated definition
create table SDATA
(
  SID bigint unsigned auto_increment
    primary key,
  FG                     varchar(100) charset utf8 default ''        not null,
  COUNTRY                varchar(100) charset utf8 default ''        not null,
  CITY                   varchar(100) charset utf8 default 'unknown' not null,
  F_DATE                 date default '0000-00-00'                   not null,
  INTERVAL_START         datetime default '0000-00-00 00:00:00'      not null,
  INTERVAL_DURATION      bigint unsigned default '0'                 not null,
  VOLUME                 decimal(10, 5) default '0.00000'            not null,
  SNAPSHOT_DATE          datetime default '0000-00-00 00:00:00'      not null,
  VERSION                date                                        null,
)
  ;

create index idx_SCD_IS_SD_FGN
  on SDATA (INTERVAL_START, SNAPSHOT_DATE, FG);

虽然Gordon Linoff的回复有效，但它可以运行33秒，持续一周的数据。有什么方法可以加快速度吗？

Answer 1

一种在数据库中运行良好的方法是相关子查询：

select t.*
from t
where t.timestamp = (select max(t2.timestamp)
                     from t t2
                     where t2.country = t.country and t2.city = t.city
                    );

为了提高性能，您需要(country, city, timestamp)上的索引。

Answer 2

试试这个

select *
from table
where timestamp = (select max(timestamp) from table)

修改

select *
from table a
where a.timestamp = (select max(timestamp) from table b where a.Country = b.country)

使用CTE编辑

with CTE
as
(
    select COUNTRY, CITY, max(INTERVAL_START) as date from SDATA group by 
    COUNTRY, CITY
)
select a.* from SDATA as a
inner join CTE as b on a.COUNTRY = b.COUNTRY and a.CITY = b.CITY and a.INTERVAL_START = date

选择所有组合的最大日期

2 个答案: