title: What are some useful SQL statements / usage patterns that should be known by all developers who may touch the Back end side of the project?
body: <p>What are some useful SQL statements that should be known by all developers who may touch the Back end side of the project?</p>
  
  <p>(<strong>Update:</strong> just like in algorithm, we know there are sorting problems, shuffling problems, and we know some solutions to them.  This question is aiming at the same thing).</p>
  
  <p>For example, one I can think of are:</p>
  
  <blockquote>
    <p>Get a list of Classes that are not
    registered by any students.  (Outer
    join and check whether the match is
    NULL, or by Get from Classes table,
    all ClassIDs which are NOT IN (a
    subquery to get all ClassIDs from the
    Registrations table))</p>
  </blockquote>
  
  <p>Are there some SQL statements that should be under the sleeve of all developers that might touch back end data?</p>