Question

I have query that is returning the follwing data:

Number  WeekNumber  Date
1111    23          9/11/12 11:01 AM
1111    23          9/11/12 11:58 AM
2222    24          9/17/12 10:14 AM
2222    24          9/18/12 9:52 AM
2222    24          9/19/12 9:46 AM
2222    24          9/20/12 9:42 AM

However what I want is to get the latest date for each week, the result should be:

Number  WeekNumber  Date
1111    23          9/11/12 11:58 AM
2222    24          9/20/12 9:42 AM

What could I use to obtain this. I have tried to use MAX(DATE) but what I am obtaining is the latest date, not the latest date for each week. I have tried with distinct too but I couldn’t make it work with the WHERE clause.

Thank you very much.

Answer 1

You say the apparent 1:1 relationship between Number and WeekNumber is an artefact of the data you posted and doesn’t really apply in your actual data. If that is the case a straightforward GROUP BY won’t provide your answer.

If you are using a flavour of RDBMS which uses analytic functions this will work:

select distinct number
       , weeknumber
       , max(date) over (partition by weeknumber) as weekly_max_date
from yourtable;

If you don’t have analytics you will need to use a correlated sub-query:

select distinct t.number
       , t.weeknumber
       , q.weekly_max_date
from yourtable t
     join ( select weeknumber, max(date) as weekly_max_date
            from yourtable
            group by weeknumber) q
     on (q.weeknumber = t.weeknumber)