%let nation=usa;
%let sasfolder=H:\My Webpage\SAS;

libname here "&sasfolder";
libname library "&sasfolder";

data student;set here.std&nation ;
data mteach;set here.mtea&nation ;
data school;set here.schl&nation ;

data STD_TEA;
merge student mteach;
by idcntry idkumi;
jun=_n_;

data here.&nation.all;
merge STD_TEA school;
by idcntry idschool;

*proc means;
*class idgrader;
*var totwgt;
run;
*proc print data=ALL3;
*where jun < 500;
*var idschool idgrader idkumi idstud;
*run;