Identify rows that already exist in same dataset and group them by date

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Identify rows that already exist in same dataset and group them by date

Hi all

I am having some trouble with a big dataset. The dataset is comprised of clients, platforms (its ids), code of platform and Period platform created.
I am trying for each period group to identify whether the clients (count them) existed previously and group them by that period and Platform. Any ideas on how to do that?

Base table looks like this:
CLIENT_ID PLAT_ID OPEN_DT SEMES PLATF
1 11 01/02/2016 Q1 16 ZEN
2 22 01/03/2016 Q1 16 SEN
4 33 01/04/2016 Q2 16 PENG
4 44 01/05/2016 Q2 16 SEN
5 55 01/06/2016 Q2 16 SEN
6 66 01/06/2016 Q2 16 SEN
4 54 01/05/2015 Q2 15 PENG
1 15 01/02/2015 Q1 16 ZEN

and I would expect a result like this:

SEMES COUNT(CLIENT_ID) Type PLATF
Q1 16 2 All --
Q1 16 1 New SEN
Q1 16 1 Existing ZEN
Q2 16 3 All --
Q2 16 1 Existing PENG
Q2 16 1 Existing SEN
Q2 16 1 New SEN
Q2 16 1 New SEN

Any help appreciated.

Thanks,
mk