On 06/30/2011 02:59 PM, Chris Campbell wrote:
Hello list,
Using PostgreSQL 9.0.2 I’m trying to write a sql statement to extract specific rows from the following data set. I’ve looked at the documentation that talks about the Max() function but the example doesn’t go far enough:
The sample fields/dataset are as follows
PaymentKey, ContactKey, PaymentDate
1, 100, 01/01/2011
2, 100, 12/30/2010
3, 100, 12/31/2010
4, 101, 01/02/2011
5, 101, 12/25/2010
What I want returned are rows grouped by contactkey showing the corresponding payment key and payment date for the record that has the max (newest) payment date. So I would want my result set to look like this:
1, 100, 01/01/2011
4, 101, 01/02/2011
I would be using this query as a sub query that is (left) joined to a master query by ContactKey
Any assistance would be greatly appreciated.
Thank you,
Chris
I am going to make some assumptions here:
1. PaymentKey is unique.
2. There can never be more than one payment for a given ContactKey on any given date (and this should be enforced by table-constraints).
If that is not true, the whole thing blows up.
One way (untested and probably slow on big tables) is with a sub-select:
select * from yourtable o where PaymentDate = (select max(PaymentDate) from yourtable i where i.ContactKey = o.ContactKey);
Probably a better way is with common table expressions:
with foo as (select PaymentKey, max(PaymentDate) as MaxPayDate from yourtable group by 1)
select * from yourtable y where exists (select 1 from foo f where (f.PaymentKey,f.MaxPayDate) = (y.PaymentKey,y.PaymentDate);
Cheers,
Steve