title: Have you extended nested sets for hierarchical data modeling involving multiple parent nodes for a child node? What are your experiences?
body: <p>I am looking to use this concept in one of my upcoming project.</p>
  
  <p>More info: <a href="http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/" rel="nofollow noreferrer">Managing Hierarchical Data in MySQL</a>.</p>
  
  <p>Please share your experiences good or bad with examples.</p>
  
  <p>I am adding more information to make it more broad:</p>
  
  <p>I have child items that can have more than one parent (example: a user can belong to city and also a group called UserDefinedRegion), which the typical hierarchical models do not support, whether it is adjacency list or nested sets.</p>
  
  <p>I am pasting the use case here for clarity:</p>
  
  <hr>
  
  <p>Background: Currently the system has a fixed hierarchy in place which is State->County->City->User</p>
  
  <ol>
  <li><p>Sales Manager logs in to the system and creates a new group which can by at the same level as City, or County.</p></li>
  <li><p>Sales Manager logs in to the system and creates a new group which can be in between State and county or County and City. </p></li>
  <li><p>Once the sales manager creates the groups, he should be able to view all the necessary reports rolled up the next day in his dashboard.</p></li>
  </ol>
  
  <hr>
  
  <p>As you can see, second point can easily be accomplished by nested sets, but not the first point, which will introduce new parents nodes for the same child node.</p>
  
  <p>So far the following solutions were proposed by stackOverflow users:</p>
  
  <ol>
  <li>Network Node structure supported by Network Database.</li>
  <li>Directed Acyclic graphs.</li>
  </ol>
  
  <p>I am definitely looking for a RDBMS solution. It looks like not many have encountered multiple parent nodes in heirarchical data models in real life.</p>