drop table if exists classes;
create table classes
(
  id int unsigned not null auto_increment,
  name varchar(50) not null,
  description tinytext,
  parent_class_id int unsigned not null default 0,
  
  
  index(parent_class_id),
  foreign key(parent_class_id) references classes(id),

  primary key(id) 
)ENGINE=InnoDB;
                       
/*
This would bee the "base" class that all other classes inherit from by default.
The problem is creating this first record with the foreign key value referring to the same primary key value, and both being created at the same time.
*/
insert into classes 
values(0,'base','description blah blah....',0);

/*
These are the following results

Query OK, 0 rows affected (0.34 sec)

Query OK, 0 rows affected (0.19 sec)

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`proj`.`classes`, CONSTRAINT `classes_ibfk_1` FOREIGN KEY (`parent_class_id`) REFERENCES `classes` (`id`))
*/

CREATE TABLE classes
( id INTEGER UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY 
, name VARCHAR(50) NOT NULL
, description TINYTEXT NULL
, parent_class_id INTEGER UNSIGNED NULL
, INDEX (parent_class_id)
, FOREIGN KEY (parent_class_id) REFERENCES classes (id)
) ENGINE=InnoDB
;
INSERT 
  INTO classes 
VALUES
     ( 0
     , 'base'
     , 'description blah blah....'
     , NULL )
;