Postgresql提取每个id的最后一行

Question

假设我有下一个数据

  id    date          another_info
  1     2014-02-01         kjkj
  1     2014-03-11         ajskj
  1     2014-05-13         kgfd
  2     2014-02-01         SADA
  3     2014-02-01         sfdg
  3     2014-06-12         fdsA

我想为每个id提取最后的信息:

  id    date          another_info
  1     2014-05-13         kgfd
  2     2014-02-01         SADA
  3     2014-06-12         fdsA

我怎么能管理呢？

Answer 1

最有效的方法是使用Postgres' distinct on运算符

select distinct on (id) id, date, another_info
from the_table
order by id, date desc;

如果您想要一个跨数据库工作的解决方案(但效率较低),您可以使用窗口函数:

select id, date, another_info
from (
  select id, date, another_info, 
         row_number() over (partition by id order by date desc) as rn
  from the_table
) t
where rn = 1
order by id;

具有窗口函数的解决方案在大多数情况下比使用子查询更快.

Answer 2

select * 
from bar 
where (id,date) in (select id,max(date) from bar group by id)

在PostgreSQL,MySQL中测试过

Answer 3

我发现这是最快的解决方案：

 SELECT t1.*
   FROM yourTable t1
     LEFT JOIN yourTable t2 ON t2.tag_id = t1.tag_id AND t2.value_time > t1.value_time
  WHERE t2.tag_id IS NULL