How to delete sub categories in codeigniter

How to delete all sub categories and their inner categories if I delete the main category ?

Table

Here if I delete the category 'computer' means how to delete their sub categories also (mouse,keyboard,virtual,touch) ?

You have one of the following options to accomplish this:

1- You setup your database to enforce referential integrity, then it's as easy as telling MySQL ON DELETE CASCADE;

Note tested, but I guess it should work.

ALTER TABLE categories
ADD FOREIGN KEY (parent_id)
REFERENCES Categories(id)
ON DELETE CASCADE;

2- Second option is calling your delete function recursively to accomplish this. example:

public function delete_category_by_id ($category_id) {
    // delete this category.
    $this->db->delete('categories', ['id' => $category_id]);
    // fetch child categories & call the same method again.
    $q = $this->db->where('parent_id', $category_id)->get('categories');
    foreach( $q->result() as $Child ) {
        $this->delete_category_by_id($Child->id);
    }
}

This would be very destructive & it might take long time depending on how many levels you have