title: How to subtract two sql datetime fields
body: <p>I have two datetime values on sql database and I want to subtract them and get the difference as a number of hours.</p>
  
  <p>So far I have created three values. Deptime and ArrTime as datetime values and here are my questions.</p>
  
  <p>1) What value shall the third table be? can it be number because I want to use it for statistics?</p>
  
  <p>2) How can I subtract the Deptime and Arrtime and store the differnce in the FlightTime?</p>
  
  <p>Thanks in advance</p>