title: Why no many-to-many relationships?
body: <p>I am learning about databases and SQL for the first time.  In the text I'm reading (Oracle 11g: SQL by Joan Casteel), it says that "many-to-many relationships can't exist in a relational database." I understand that we are to avoid them, and I understand how to create a bridging entity to eliminate them, but I am trying to fully understand the statement "can't exist."</p>
  
  <p>Is it actually physically impossible to have a many-to-many relationship represented?</p>
  
  <p>Or is it just very inefficient since it leads to a lot of data duplication?</p>
  
  <p>It seems to me to be the latter case, and the bridging entity minimizes the duplicated data. But maybe I'm missing something? I haven't found a concrete reason (or better yet an example) that explains why to avoid the many-to-many relationship, either in the text or anywhere else I've searched.  I've been searching all day and only finding the same information repeated: "don't do it, and use a bridging entity instead."  But I like to ask why. :-)</p>
  
  <p>Thanks! </p>