title: Searching for missing or duplicated values in composite keys
body: <p>Let's say I have a table with a two-part composite key like the following:</p>
  <pre>
  Key1  Key2  
  a     char1  
  a     char2  
  a     char3  
  a     char4  
  a     char5
  b     char1 
  etc.
  </pre>
  <p>I need to check if there is a value form Key1 with missing or duplicated corresponding values from Key2 (i.e. values from key1 without 5 specific values from key2). I'm new to sql, thanks in advance for help.</p>