Using ORDER BY and GROUP BY together

Question

My table looks like this (and I'm using MySQL):

m_id | v_id | timestamp
------------------------
6    |   1  | 1333635317
34   |   1  | 1333635323
34   |   1  | 1333635336
6    |   1  | 1333635343
6    |   1  | 1333635349

My target is to take each m_id one time, and order by the highest timestamp.

The result should be:

m_id | v_id | timestamp
------------------------
6    |   1  | 1333635343
34   |   1  | 1333635336

And i wrote this query:

SELECT * FROM table GROUP BY m_id ORDER BY timestamp DESC

But, the results are:

m_id | v_id | timestamp
------------------------
34   |   1  | 1333635323
6    |   1  | 1333635317

I think it causes because it first does GROUP_BY and then ORDER the results.

Any ideas? Thank you.

Answer 1

One way to do this that correctly uses group by:

select l.* 
from table l
inner join (
  select 
    m_id, max(timestamp) as latest 
  from table 
  group by m_id
) r
  on l.timestamp = r.latest and l.m_id = r.m_id
order by timestamp desc

How this works:

• selects the latest timestamp for each distinct m_id in the subquery
• only selects rows from table that match a row from the subquery (this operation -- where a join is performed, but no columns are selected from the second table, it's just used as a filter -- is known as a "semijoin" in case you were curious)
• orders the rows

Answer 2

If you really don't care about which timestamp you'll get and your v_id is always the same for a given m_i you can do the following:

select m_id, v_id, max(timestamp) from table
group by m_id, v_id
order by timestamp desc

Now, if the v_id changes for a given m_id then you should do the following

select t1.* from table t1
left join table t2 on t1.m_id = t2.m_id and t1.timestamp < t2.timestamp
where t2.timestamp is null
order by t1.timestamp desc

Answer 3

You can try this

 SELECT tbl.* FROM (SELECT * FROM table ORDER BY timestamp DESC) as tbl
 GROUP BY tbl.m_id  

Answer 4

SQL>

SELECT interview.qtrcode QTR, interview.companyname "Company Name", interview.division Division 
FROM interview 
JOIN jobsdev.employer 
    ON (interview.companyname = employer.companyname AND employer.zipcode like '100%')
GROUP BY interview.qtrcode, interview.companyname, interview.division
ORDER BY interview.qtrcode;

Answer 5

Here is the simplest solution

select m_id,v_id,max(timestamp) from table group by m_id;

Group by m_id but get max of timestamp for each m_id.

Answer 6

Just you need to desc with asc. Write the query like below. It will return the values in ascending order.

SELECT * FROM table GROUP BY m_id ORDER BY m_id asc;

Answer 7

Why make it so complicated? This worked.

SELECT m_id,v_id,MAX(TIMESTAMP) AS TIME
 FROM table_name 
 GROUP BY m_id