text: --test alter using to alter default value of a column 
  
  create class tb2(
  	col1 char(8)
  );
  
  
  create class tb1(
  	col1 int,
  	col2 tb2
  );
  
  
  alter class tb2 modify col1 char(8) default 'aaa'; 
  
  drop class tb2;
  drop class tb1;
  
  