SAS/SQL group by and keeping all rows - plsql

Question

I have a table like this, observing the behavior of some accounts in time, here two accounts with acc_ids 1 and 22:
acc_id date mob
1 Dec 13 -1
1 Jan 14 0
1 Feb 14 1
1 Mar 14 2
22 Mar 14 10
22 Apr 14 11
22 May 14 12
I would like to create a column orig_date that would be equal to date if mob=0 and to minimum date by acc_id group if there is no mob=0 for that acc_id.
Therefore the expected output is:
acc_id date mob orig_date
1 Dec 13 -1 Jan 14
1 Jan 14 0 Jan 14
1 Feb 14 1 Jan 14
1 Mar 14 2 Jan 14
22 Mar 14 10 Mar 14
22 Apr 14 11 Mar 14
22 May 14 12 Mar 14
The second account does not have mob=0 observation, therefore orig_date is set to min(date) by group.
Is there some way how to achieve this in SAS, preferably by one proc sql step?

Answer 1

Seems pretty simple. Just calculate the min date in two ways and use coalesce() to pick the one you want.
First let's turn your printout into an actual dataset.
data have ;
input acc_id date :anydtdte. mob ;
format date date9.;
cards;
1 Dec13 -1
1 Jan14 0
1 Feb14 1
1 Mar14 2
22 Mar14 10
22 Apr14 11
22 May14 12
;
To find the DATE when MOB=0 use a CAsE clause. PROC SQL will automatically remerge the MIN() aggregate results calculated at the ACC_ID level back onto all of the detail rows.
proc sql ;
create table want as
select *
, coalesce( min(case when mob=0 then date else . end)
, min(date)
) as orig_date format=date9.
from have
group by acc_id
order by acc_id, date
;
quit;
Result:
Obs acc_id date mob orig_date
1 1 01DEC2013 -1 01JAN2014
2 1 01JAN2014 0 01JAN2014
3 1 01FEB2014 1 01JAN2014
4 1 01MAR2014 2 01JAN2014
5 22 01MAR2014 10 01MAR2014
6 22 01APR2014 11 01MAR2014
7 22 01MAY2014 12 01MAR2014

Answer 2

Here is a data step approach
data have;
input acc_id date $ mob;
datalines;
1 Dec13 -1
1 Jan14 0
1 Feb14 1
1 Mar14 2
22 Mar14 10
22 Apr14 11
22 May14 12
;
data want;
do until (last.acc_id);
set have;
by acc_id;
if first.acc_id then orig_date=date;
if mob=0 then orig_date=date;
end;
do until (last.acc_id);
set have;
by acc_id;
output;
end;
run;