Design de estrutura de banco de dados com quantidades variáveis ​​de campos

create table property_types (
  property_id int primary key,
  name varchar,
  -- more info here
);

create table items (
  item_id int primary key,
  -- your item table
);

-- this table links a property value with an item
create table item_properties (
  item_id int,
  property_id int,
  property_value varchar,
  foreign key fk_item (item_id) references items (item_id),
  foreign key fk_property (property_id) references properties (property_id)
);