Is contract_no numeric or character ?
If it is numeric:
    proc sql ;
      select distinct contract_no into : contract_no separated by ','
      from aaa;
    quit;


If it is character:
    proc sql ;
      select distinct "'"||strip(contract_no)||"'" into : contract_no separated by ','
      from aaa;
    quit;


GOOD LUCK

It is character type variable. Thank you so much.Pity,micro variable 'contract_no' is unavilable in the 'proc sql' step connect to oracle. Like:

Did you create macro varible 'contract_no' as ??
   proc sql ;
       select distinct "'"||strip(contract_no)||"'" into : contract_no separated by ','
       from aaa;
     quit;
If so, please use %put &contract_no to display here.