Question

I have a table recording the amount of data transferred by a given service on a given date. One record is entered daily for a given service.

I’d like to be able to retrieve the most recent amount for a set of services.

Example data set:

serviceId | amount | date
-------------------------------
1         | 8      | 2010-04-12   
2         | 11     | 2010-04-12  
2         | 14     | 2010-04-11  
3         | 9      | 2010-04-11  
1         | 6      | 2010-04-10  
2         | 5      | 2010-04-10  
3         | 22     | 2010-04-10  
4         | 17     | 2010-04-19

Desired response (service ids 1,2,3):

serviceId | amount | date
-------------------------------
1         | 8      | 2010-04-12   
2         | 11     | 2010-04-12  
3         | 9      | 2010-04-11 

Desired response (service ids 2, 4):

serviceId | amount | date
-------------------------------
2         | 11     | 2010-04-12  
4         | 17     | 2010-04-19

This retrieves the equivalent as running the following once per serviceId:

SELECT serviceId, amount, date
FROM table
WHERE serviceId = <given serviceId>
ORDER BY date DESC
LIMIT 0,1

I understand how I can retrieve the data I want in X queries. I’m interested to see how I can retrieve the same data using either a single query or at the very least less than X queries.

I’m very interested to see what might be the most efficient approach. The table currently contains 28809 records.

I appreciate that there are other questions that cover selecting the most recent set of records. I have examined three such questions but have been unable to apply the solutions to my problem.

Answer 1

select m.*
from (
    select serviceId, max(date) as MaxDate
    from MyTable
    group by serviceId
) mm
inner join MyTable m on mm.serviceId = m.serviceId and mm.MaxDate = m.date

If you wish to filter by serviceId, you can do:

select m.*
from (
    select serviceId, max(date) as MaxDate
    from MyTable
    where serviceId in (1, 2, 3)
    group by serviceId
) mm
inner join MyTable m on mm.serviceId = m.serviceId and mm.MaxDate = m.date