title: How to check a data row in mysql with triggers?
body: <p>I want to be sure, that there are no children of children in my datatable. 
  So if I have a parent item A, and a child item B (B.parent = A), and I try to insert a child item C to the item B (C.parent = B), this trigger have to prevent it and set the parent_id of C to A (C.parent = A). I need only 2 levels in my table (parent-child), and no grandpas.</p>
  
  <p>There is my sample, that doesn't work:</p>
  
  <pre><code>DELIMITER //
  CREATE TRIGGER parent_control BEFORE insert ON reports
  FOR EACH ROW BEGIN
  IF new.parent_id is not null THEN
    set @parent_parent_id = new.parent_id;
    SELECT parent_id FROM reports INTO parent_parent_id WHERE report_id = new.parent_id;
    IF @parent_parent_id is not null THEN
      SET new.parent_id = @parent_parent_id;
    END IF;
  END IF;
  END; 
  </code></pre>
  
  <p>It says: #1327 - Undeclared variable: parent_parent_id</p>