Question

I have 2 Tables — Table 1 is a master file and Table 2 is an activity file.

The relationship is 1-to-Many.

I am producing a report and all I want to return is every master file row joined to only the last activity row for the related master id.

I am unsure on how to request the last activity row. My code below returns every activity row (rightfully so).

Thank you for your help.

    SELECT *
    FROM master_file AS master
    INNER JOIN activity_file AS activity ON activity.id = master.id
    ORDER BY master.display_name

The activity file has a column called entry_date. It is a date and time stamp recording every activity. I simply want to select the last entry_date.

For example:

Table 2 – Activity looks like this

ID ACTIVITY ENTRY_DATE
1 Update 2012-08-01 09:00:00
1 Edit 2012-08-01 13:45:15
3 Create 2012-07-15 10:09:52
3 Delete 2012-07-22 23:02:00
3 Add 2012-08-05 04:33:00
4 Edit 2012-08-03 15:12:00

Answer 1

One standard way to solve this is to create an inline view that finds the last entry_date per ID.

SELECT *
    FROM master_file AS master
    LEFT JOIN activity_file AS activity ON activity.id = master.id
    LEFT JOIN (select id , max(entry_date) entry_date
                From activity_file 
                group by id) last_activity
    ON activity_file.id = last_activity.id
      and  activity_file.entry_date= last_activity.entry_date

    ORDER BY master.display_name

The one problem with this approach is that you may have more than one record with max(entry_date) for a given id. You’ll either need to have your business rules handle this (e.g. simply display more than one record for that case) or you’ll need to figure out a tie breaker. The last thing you want is to make it non-deterministic