title: SQL query on two tables - return rows in one table that don't have entries in the other
body: <p>I have two database tables, Categories and SuperCategories, for an inventory control system I'm working on:  </p>
  
  <p>Categories:  ID_Category, CategoryName  </p>
  
  <p>SuperCategories:  ID_SuperCategory, CategoryID, SuperCategoryID  </p>
  
  <p>I'm putting category-subcategory relationships into the SuperCategories table.  I'm putting all categories into the Categories table.</p>
  
  <p>Here is an example:  </p>
  
  <pre>
      Categories:
      ID_Category CategoryName
      1           Box
      2           Red Box
      3           Blue Box
      4           Blue Plastic Box
      5           Can
      6           Tin Can  
  </pre>
  
  <pre>    
      SuperCategories:
      ID_Super CategoryID SuperCategoryID
      1        2          1
      2        3          1
      3        4          3
      4        6          5
  </pre>
  
  <p>CategoryID and SuperCategoryID relate back to the primary key ID_Category in the Categories table.</p>
  
  <p>What I would like is a query that returns all of the category names that are not parents of any other categories:</p>
  
  <p>Red Box<br>
  Blue Plastic Box<br>
  Tin Can  </p>
  
  <p>This amounts to finding all values of ID_Category that do not show up in the SuperCategoryID column (2, 4, and 6), but I'm having trouble writing the SQL.</p>
  
  <p>I'm using VB6 to query an Access 2000 database.</p>
  
  <p>Any help is appreciated.  Thanks!</p>
  
  <p><strong>EDIT:</strong> I voted up everyone's answer that gave me something that worked.  I accepted the answer that I felt was the most instructive.  Thanks again for your help!</p>