title: Normalization; how to judiciously determine if a field is dependent on a prime attribute for its existence?
body: <p>Mainly concerning partial dependencies (2nd Normal Form)</p>
  
  <p>Say, you're designing a database for a train service, and would like to keep track of its data in relation to its scheduled and actual departure and arrival times. Now, given that the scheduled departure time is part of the composite key for a particular table, consider the following:</p>
  
  <p>Would you say that even for the scheduled arrival time attribute, is that attribute actually dependent on the scheduled departure time for its existence? Arguably, given that there was substantial research and work gone into determining the travel duration between two train stations, and thus, one could determine a very close approximate for the arrival time, given its scheduled departure time (the key/prime attribute), does that mean the scheduled arrival time is dependent on the scheduled departure time?</p>
  
  <p>Similarly, for the actual departure and arrival times, would you say either of these two attributes depends on the scheduled departure time? How can you be judicious about this as a database designer?</p>