text: create table `a` (
      `id` int primary key auto_increment,
      `name` varchar(255) not null
  );
  create table `b` (
      `id` int primary key auto_increment,
      `a_id` int not null,
      `name` varchar(255) not null
  );
  create table `c` (
      `id` int primary key auto_increment,
      `b_id` int not null,
      `d_id` int,
      `name` varchar(255) not null
  );
  create table `d` (
      `id` int primary key auto_increment,
      `name` varchar(255) not null
  );
  create table `e` (
      `id` int primary key auto_increment,
      `d_id` int not null,
      `sub_id` int not null,
      `name` varchar(255) not null
  );
  