SAS Count number of changes between consecutive time periods - sas

Question

Proc SQL Version=9.4. No windows functions to use.
There are client id, time period(month), amount and corresponding class.
client_id data_period amount class
1 200801 30000 2
2 200801 17000 1
3 200801 9000 1
1 200802 30000 2
2 200802 55555 2
3 200802 11000 2
Threshold amount = 20 000.
amount > 20k gives class = 2, amount <= 20k makes class = 1
client_id = 1, amount and class are the same for 200801 and 200802.
client_id = 2, amount gets higher from 17k to 55.5k, class change is correct, from 1 to 2.
client_id =3, amount changed within the same class 1 (<20K), but class changed incorrectly.
Desired result is
client_id oldDate newDate AmtOld AmtNew ClassOld ClassNew Good Bad
2 200801 200802 17000 55555 1 2 1 0
3 200801 200802 9000 11000 1 1 0 1
I tried to applied self join to get all the differences btw data periods, but there are too many rows in output. Data below is not from example above, real numbers.
client_id oldDate newDate AmtOld AmtNew ClassOld ClassNew
A001687463 200808 200802 -5613 1690386 I03 I04
A001687463 200807 200802 -5613 1690386 I03 I04
A001687463 200806 200802 -5613 1690386 I03 I04
A001687463 200805 200802 -5613 1690386 I03 I04
PROC SQL;
CREATE TABLE WORK.'Q'n AS
SELECT distinct
t1.client_id, t1.data_period as oldDate, t2.data_period as newDate, t1.amount as expAmtOld, t2.amount as expAmtNew, t1.class as classOld, t2.class as classNew
FROM WORK.'E'n t1, WORK.'E'n t2
where
t1.client_id = t2.client_id and
t1.amount <> t2.amount
order by t1.client_id;

Answer 1

Do not attempt to do sequential processing using SQL. It is not built for that.
It should be easy to do in a data step. For example let's convert your printout into an actual SAS dataset so we have something to code with.
data have ;
input client_id data_period amount class ;
cards;
1 200801 30000 2
2 200801 17000 1
3 200801 9000 1
1 200802 30000 2
2 200802 55555 2
3 200802 11000 2
;
And let's sort it by client and period.
proc sort data=have ;
by client_id data_period ;
run;
Now just set the data and use the LAG() function to get the previous values.
Not sure what you definition of GOOD and BAD were so I just created new class variables based on your rule of 20K.
data want ;
set have ;
by client_id;
old_period = lag(data_period);
old_class = lag(class);
newclass = 1 + (amount > 20000) ;
old_newclass = lag(newclass);
if first.client_id then call missing(of old_:);
bad = (class ne newclass) or (old_newclass ne old_class) ;
run;
So here are the results.
client_ data_ old_ old_ old_
id period amount class period class newclass newclass bad
1 200801 30000 2 . . 2 . 0
1 200802 30000 2 200801 2 2 2 0
2 200801 17000 1 . . 1 . 0
2 200802 55555 2 200801 1 2 1 0
3 200801 9000 1 . . 1 . 0
3 200802 11000 2 200801 1 1 1 1