title: Analysis of SQL Schema: what is the purpose of loop in ProductCategory of AdventureWorks?
body: <p>I want to understand the purpose of the loop in the ProductCategory table. AdventureWorks is a sample database <a href="https://msdn.microsoft.com/en-us/library/ms124501(v=sql.100).aspx" rel="nofollow noreferrer">here</a> provided by Microsoft. Why is there a loop in the ProductCategory table of AdventureWorks?</p>
  
  <p><strong>Diagram from Microsoft SQL Server Management Studio</strong> where loop is on the right</p>
  
  <p><a href="https://i.stack.imgur.com/rMv70.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/rMv70.png" alt="enter image description here"></a></p>
  
  <p><strong>Diagram (now called Reference Diagram, in Hirarchy mode, generated by Db Vis)</strong> where loop is on the bottom-right</p>
  
  <p><a href="https://i.stack.imgur.com/YYr1N.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YYr1N.png" alt="enter image description here"></a></p>