create table Product (
   Id int not null primary key
)

create table Category (
   Id int not null promary key
)

create table SubCategory (
   CategoryId not null foreign key references Category(Id),
   Id int not null,
   constraint PK_SubCategory primary key (CategoryId, Id)
)

create table ProductCategory (
   ProductId int not null foreign key references Product(Id),
   CategoryId int not null foreign key references Category(Id),
   SubCategoryId int null,
   constraint PK_ProductCategory primary key (ProductId, CategoryId),
   constraint FK_ProductCategory_SubCategory foreign key (CategoryId, SubCategoryId) 
          references SubCategory(CategoryId, Id)
)