Question

我有一个带有重复条目的postgres数据库。我想显示created_by列和

id | name    | created_on
1  | 'Hello' | 3/29
2  | 'Hey'   | 4/3
3  | 'Hello' | 3/31
4  | 'Hey'   | 4/1

由于'Hello'和'Hey'是重复的，我想形成一个查询列出以下内容：

id | name    | created_on
2  | 'Hey'   | 4/3
3  | 'Hello' | 3/31

我怎么能形成那个SQL查询？

实际上我尝试在单个查询中使用'DISTINCT ON'和'ORDER BY'（使用JOIN）它会给我以下错误：

SELECT DISTINCT ON expressions must match initial ORDER BY expressions

提前致谢

Answer 1

试试这个：

SELECT t1.*
FROM tablename AS t1
INNER JOIN
(
  SELECT name, MAX(created_on) AS MaxDate
  FROM tablename 
  GROUP BY name
) AS t2  ON t1.name       = t2.name
        AND t1.created_on = t2.maxdate;

SQL Fidle Demo

Answer 2

尝试此查询

select 
  a.* 
from 
  table1 a
inner join 
  (select 
     name, 
     max(created_on) as date 
  from 
     table1 
  group by 
     name)b
on 
  a.name = b.name AND 
  a.created_on = b.date

FIDDLE

Answer 3

SQL Fiddle

select distinct on (name)
    id, name, created_on
from tablename
order by name, created_on desc

如果您需要按ID订购：

select *
from (
    select distinct on (name)
        id, name, created_on
    from tablename
    order by name, created_on desc
) s
order by id

Postgres：重复发生时是否可以只显示一个条目？

3 个答案:

SQL Fidle Demo