Calculate average of the last x years - sas

Question

I have the following data
Date value_idx
2002-01-31 .
2002-01-31 24.533
2002-01-31 26.50
2018-02-28 25.2124
2019-09-12 22.251
2019-01-31 24.214
2019-05-21 25.241
2019-05-21 .
2020-05-21 25.241
2020-05-21 23.232
I would need to calculate the average of value_idx of the last 3 years and 7 years.
I tried first to calculate it as follows:
proc sql;
create table table1 as
select date, avg(value_idx) as avg_value_idx
from table
group by date;
quit;
The problem is that I do not know how to calculate the average of value_idx not per each month but for the last two years. So I think I should extract the year, group by that, and then calculate the average.
I hope someone of you can help me with this.

Answer 1

You can use CASE to decide which records contribute to which MEAN. You need to clarify what you mean by last 2 or last 7 years. This code will find the value of the maximum date and then compare the year of that date to the year of the other dates.
select
mean(case when year(max_date)-year(date) < 2 then value_idx else . end) as mean_yr2
,mean(case when year(max_date)-year(date) < 7 then value_idx else . end) as mean_yr7
from have,(select max(date) as max_date from have)
;
Results
mean_yr2 mean_yr7
------------------
24.0358 24.2319

Answer 2

The best way to do this sort of thing in SAS is with native PROCs, as they have a lot of functionality related to grouping.
In this case, we use multilabel formats to control the grouping. I assume you mean 'Last Three Years' as in calendar 2018/2019/2020 and 'Last Seven Years' as calendar 2014-2020. Presumably you can see how to modify this for other time periods - so long as you aren't trying to make the time period relative to each data point.
We create a format that uses the MULTILABEL option (which allows data points to fall in multiple categories), and the NOTSORTED option (to allow us to force the ordering of the labels, otherwise SEVEN is earlier than THREE).
Then, we use it in PROC TABULATE, enabling it with MLF (MultiLabel Format) and preloadfmt order=data which again keeps the ordering correct. This produces a report with the two averages only.
data have;
informat date yymmdd10.;
input Date value_idx;
datalines;
2002-01-31 .
2002-01-31 24.533
2002-01-31 26.50
2017-02-28 25.2124
2017-09-12 22.251
2018-01-31 24.214
2018-05-21 25.241
2019-05-21 .
2020-05-21 25.241
2020-05-21 23.232
;;;;
run;
proc format;
value yeartabfmt (multilabel notsorted)
'01JAN2018'd-'31DEC2020'd = 'Last Three Years'
'01JAN2014'd-'31DEC2020'd = 'Last Seven Years'
other=' '
;
quit;
proc tabulate data=have;
class date/mlf preloadfmt order=data;
var value_idx;
format date yeartabfmt.;
tables date,value_idx*mean;
run;