Conditionally add a label column based on dates and flags of another dataset

[NewUsrStat]

Hi guys, 

suppose to have the following two datasets: 

The first one contains a full list of dates per ID and an index that indicates if the patient has a comorbidity (1) or not (0). Then, there's another dataset that contains a subset of dates from the first dataset and a RefDate, 1 or 0. What I would like to do is the following: 

for each ID:

• if the comorbidity in DB1 = 1(!) before or =  to the Start_Therapy where in DB2 RefDate = 1 then add an Index in DB1 equal to 1 otherwise the Index should be 0 regardless if in DB1 the comorbidity = 1.
• if the comorbidity in DB1 = 0 it should remain 0.

Thank you in advance!

Desired output = DB3

data DB1;
  input ID Start_Therapy Comorbidity;
  format Start_Therapy date9.;
cards;
0001 01OCT2015  1
0001 06DEC2016  1
0001 08NOV2020  1
0002 11JAN2014  0
0002 16JUN2014  0
0002 14MAY2015  1
0002 30JUN2015  1
0002 25FEB2016  0
0003 11SEP2012  0
0003 24AUG2014  1
0003 10DEC2014  1
0004 03JAN2014  0
0004 09FEB2014  1
0004 03AUG2015  1
0004 18MAY2016  0
;


data DB2;
  input ID Start_Therapy  RefDate;
  format Start_Therapy date9.;
cards;
0001 06DEC2016  1
0001 08NOV2020  0
0002 16JUN2014  1
0002 30JUN2015  0
0003 24AUG2014  1
0003 10DEC2014  0
0004 03AUG2015  1
;

data DB3;
  input ID  Start_Therapy  Comorbidity Index;
  format Start_Therapy date9.;
cards;
0001 01OCT2015  1  1
0001 06DEC2016  1  1
0001 08NOV2020  1  0
0002 11JAN2014  0  1
0002 16JUN2014  0  1
0002 14MAY2015  1  0
0002 30JUN2015  1  0
0002 25FEB2016  0  0
0003 11SEP2012  0  0
0003 24AUG2014  1  1
0003 10DEC2014  1  0
0004 03JAN2014  0  0
0004 09FEB2014  1  1
0004 03AUG2015  1  1
0004 18MAY2016  0  0
;

[PaigeMiller]

Your title talks about creating a level. The rest of your post never mentions creating a label. Please clarify.

[NewUsrStat]

I'm sorry. It is intended as a label (new) column/index column