Task Group 1 contains
|-- Simple Task 1
|-- Simple task 2
|-- Task Group 1a contains
    |-- Simple Task 3
    |-- Simple Task 4
    |-- Task Group 1b contains
        |--Simple task 5

CREATE TABLE `task_group` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(45) DEFAULT NULL,
  `status` varchar(45) DEFAULT NULL,
  `processing_type` varchar(45) DEFAULT NULL,
  `notes` varchar(255) DEFAULT NULL,
  `parent_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_parent_idx` (`parent_id`),
  CONSTRAINT `fk_parent` FOREIGN KEY (`parent_id`) REFERENCES `task_group` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8

TaskTrees
  ParentTaskId   references Tasks.TaskId
  ChildTaskId    references Tasks.TaskId

Tasks
  TaskId
  TaskType      simple or group

GroupTasks
  TaskId   references Tasks.TaskId
  <group specific columns>

SimpleTasks
  TaskId   references Tasks.TaskId
  <other specific columns>