选择最大日期之和

Question

I have some sites that I collect analytics data from, each site has data for every country, not all the sites get data every month.

I'd like to get the sum total of visits from all the sites in their last set of data (which may not have the same date).

I can't use MAX() in the where clause as that's invalid.

I've tried storing the last date as a variable but that doesn't work:

select sites.id, sites.name, SUM(data.visits), @start := MAX(data.start_date)
inner join data on data.profile_id = sites.profile_id
where data.start_date = @start
group by sites.profile_id

I've also played around with sub queries but just not quite got it right yet.

Answer 1

Use a subquery to get the max date and join that in:

select sites.id, sites.name, SUM(data.visits), dp.maxdate
from sites inner join
     data
     on data.profile_id = sites.profile_id inner join
     (select profile_id, MAX(start_date) as maxdate
      from data
      group by profile_id
     ) dp
     on data.profile_id = dp.profile_id and data.start_date = dp.maxdate
group by sites.profile_id, sites.name, dp.maxdate