title: How to detect interactions with SQL?
body: <p>I have a SQL table that keeps track of whether entities have interacted. So, for example, each row has a unique key, a date of interaction, an entity, and a group number. If two entities have been in the same group ever, they have interacted. </p>
  
  <p>I have another table in which these entities are stored (with a unique identifier, a name, etc). </p>
  
  <p>I need a query that tells me who hasn't interacted with whom, preferably in two columns, with the first being a single entity, and the second being a list of all of the entities it hasn't interacted with. </p>
  
  <p>Is this possible? </p>
  
  <p>Thanks in advance - my SQL chops are quite rough!</p>