Left join PROC SQL using threshold date - sas

Question

I am hoping you can help me! Please help!!!!
I am in SAS using PROC SQL and I have datasets A and B with different measurements (relating to patient's health) as follows:
Dataset A
ID Date measurement_a
1 20JUN2013 52.3
1 12JUL2013 65.6
1 28NOV2014 37.4
1 02DEC2014 61.3
1 22SEP2015 40.5
1 15OCT2015 60.5
2 03JUN2011 46.5
2 19JUL2011 54.1
2 29OCT2012 53.6
...
Dataset B
ID Date measurement_b
1 21MAR2007 43
1 13JUL2007 45
1 07APR2009 47
1 14MAY2009 46
1 16FEB2012 42
1 27AUG2012 53
1 12DEC2012 58
1 20JUN2013 56
1 10DEC2013 53
1 23MAY2014 49
1 17SEP2014 44
1 23SEP2015 40
2 16DEC2011 58
2 22AUG2012 54
2 20FEB2013 56
2 29MAY2013 53
...
What I am looking for is that if the date in Dataset B is within 6 months of the date in Dataset a, then a new variable called "time" will be added, saying 1,2,3,etc. for how many ever match with ** only measurement_a** length (in other words, I do not need to retain values of measurement_b if it does not match the date in Dataset a. Here is an example of what I mean:
Desired result/dataset:
ID Time measurement_a measurement_b
1 1 52.3 56 (Dataset B Date = 20JUN2013 - Matched exactly)
1 2 65.6 53 (Dataset B date = 10DEC2013 - Within six months of 12JUL2013 [Dataset A Date])
1 3 37.4 44 (Dataset B date = 17SEP2014 - Within six months of 28NOV2014 [Dataset A Date])
1 4 61.3 . (because 17SEP2014 [Dataset B] is closest to 28NOV2014 [Dataset A])
1 5 40.5 40 (because 23SEP2015 [Dataset B] is closest to 22SEP2015 [Dataset A])
1 6 60.5 . (No date in Dataset B that is within 6 months of Date in Dataset A [15OCT2015])
2 1 46.5 . (See below)
2 2 54.1 58 (because 03JUL2011 [Dataset B] is closest to 19JUL2011 [Dataset A])
2 3 53.6 54 (Dataset B date = 22AUG2012 - Within 6 months of Dataset A date = 29OCT2012)
...
I have joined on ID but the times is proving difficult. I know it could be the difference in months in the "where" statement in the following code:
PROC SQL;
CREATE TABLE join_test as
SELECT * FROM data_a as a
LEFT_JOIN data_b as b
ON a.id = b.id
WHERE days(a.Date - b.Date) <= 180 ;
QUIT;
But this does not do the trick.
Can some please help me?
I really appreciate it. Thanks in advance.

Answer 1

In the join criteria add the use of the SAS function INTCK to compute the number of month intervals between the two date values. Proc SQL does not have a way to introduce a serial count value, so you will have to add that in a subsequent step. A LEFT JOIN will create a result set with every id/date in table A.
Example:
The columns a.date, b_date and c_months_apart were added to show how the join works. You can safely remove them from the select.
proc sql;
create table stage1 as
select
a.id
, a.date
, a.measurement_a
, b.measurement_b
, b.date as b_date
, intck('month', a.date, b.date, 'C') as c_months_apart
from
a left join b
on a.id = b.id
and intck('month', a.date, b.date, 'C') between 0 and 6
order by a.id, a.date, b.date
;
data want;
set stage1;
by id;
if first.id then time=1; else time+1;
run;
Output (want)
measurement_ measurement_ c_months_
ID Date a b b_date apart time
1 20JUN2013 52.3 56 20JUN2013 0 1
1 20JUN2013 52.3 53 10DEC2013 5 2
1 12JUL2013 65.6 53 10DEC2013 4 3
1 28NOV2014 37.4 . . . 4
1 02DEC2014 61.3 . . . 5
1 22SEP2015 40.5 40 23SEP2015 0 6
1 15OCT2015 60.5 . . . 7
2 03JUN2011 46.5 58 16DEC2011 6 1
2 19JUL2011 54.1 58 16DEC2011 4 2
2 29OCT2012 53.6 56 20FEB2013 3 3