Karun,
It is probably easiest to answer your questions with an example that only uses sas datasets so that you can see what the data looks like in a data step.
First, here are two datasteps that create files which I think are like the ones you have, other than that the second one is really a tab within an Excel workbook:
data myDatasetA;
informat myDate yyq6.;
format myDate yyq6.;
input myDate x;
cards;
2010Q1 1
2010Q2 1
2010Q3 1
2010Q4 1
;
run;
data XL_A;
informat myDate yyq6.;
format myDate yyq6.;
input myDate x;
cards;
2010Q1 2
2010Q3 2
2010Q4 2
2011Q1 2
2011Q2 2
2011Q3 2
2011Q4 2
;
run;
/*next, here is the code, but modified to work with 2 sas datasets, rather than expecting one to be from Excel */
data want (drop=maxdate);
retain maxdate;
set myDatasetA (in=inA) XL_A;
if inA then maxdate=max(maxdate,myDate);
else if myDate gt maxdate;
run;
/* the resulting file will look like:
myDate x
2010Q1 1
2010Q2 1
2010Q3 1
2010Q4 1
2011Q1 2
2011Q2 2
2011Q3 2
2011Q4 2
Thus, to answer your questions:
1. The data in myDatasetA doesn't get changed .. only new records are added to it
2. Before file XL_A is even read, maxdate was set to equal the maximum date present in the file myDatasetA. It was retained so that it would be available at that time when the records from XL_A were read
3. The format of myDate shouldn't matter as long as it was input correctly to represent a SAS date
4. Yes, you appear to understand the first part. The second part is a SAS statement that selects records from XL_A if, and only if, their value for myDate is greater than maxdate. An easy way to see that work is with another example. e.g.:
*/
data test;
set sashelp.class;
if sex eq "M";
run;
/* That code will ONLY select records that have a value of "M" for the variable called sex */
