Select distinct rows whilst grouping by max value

问题

I currently have the following table:

ID   |  Name    |  EventTime            |  State
1001 |  User 1  |  2013/07/22 00:00:05  |  15
1002 |  User 2  |  2013/07/23 00:10:00  |  100
1003 |  User 3  |  2013/07/23 06:15:31  |  35
1001 |  User 1  |  2013/07/23 07:13:00  |  21
1001 |  User 1  |  2013/07/23 08:15:00  |  25
1003 |  User 3  |  2013/07/23 10:00:00  |  22
1002 |  User 2  |  2013/07/23 09:18:21  |  50

What I need is the state for each distinct userid from the last eventtime similar to below:

ID   |  Name    |  EventTime            |  State
1001 |  User 1  |  2013/07/23 08:15:00  |  25
1003 |  User 3  |  2013/07/23 10:00:00  |  22
1002 |  User 2  |  2013/07/23 09:18:21  |  50

I need something similar to the following but I can't quite get what I need.

SELECT ID, Name, max(EventTime), State
FROM MyTable
GROUP BY ID

回答1:

In databases that support analytic functions, you could use row_number():

select  *
from    (
        select  row_number() over (partition by ID 
                                   order by EventTime desc) as rn
        ,       *
        from    YourTable
        ) as SubQueryAlias
where   rn = 1

回答2:

SELECT
ID, Name, EventTime, State
FROM
MyTable mt
WHERE EventTime = (SELECT MAX(EventTime) FROM MyTable sq WHERE mt.ID = sq.ID)

回答3:

You did not specify what database you are using but you should be able to use an aggregate function in a subquery to get the max event time for each id:

select t1.id,
  t1.name,
  t1.eventtime,
  t1.state
from mytable t1
inner join
(
  select max(eventtime) eventtime, id
  from mytable
  group by id
) t2
  on t1.id = t2.id
  and t1.eventtime = t2.eventtime
order by t1.id;

See SQL Fiddle with Demo

回答4:

You can try this:-

SELECT ID, Name, EventTime, State
FROM mytable mm Where EventTime IN (Select MAX(EventTime) from mytable mt where mt.id=mm.id)

SQL FIDDLE

来源：https://stackoverflow.com/questions/17814961/select-distinct-rows-whilst-grouping-by-max-value