data combined;
  set sashelp.prdsale(in=in_prdsale)
      sashelp.prdsal2(in=in_prdsal2)
      sashelp.prdsal3(in=in_prdsal3);
  length Source $ 32;
  if in_prdsale then source="PRDSALE";
  else if in_prdsal2 then source="PRDSAL2";
  else if in_prdsal3 then source="PRDSAL3";
run;
proc freq data=combined;
  tables Source;
  title1 "Source Data Sets for Combined Data";
run;