text: -- [er]create vclass attribute using not null and unique constraints
  
  create class picture
  (caption set string,
   image string);
   
   insert INTO picture VALUES ({'s'},'s');
   
   create vclass meal
  class attribute (meal_type string unique not null)
  (menu picture);
   
   select * FROM db_attribute WHere class_name='meal' order by 1;
   
   
   SELECT * FROM meal;
   
   
   DROP picture;
   
   DROP meal;
  