title: SQL get date when specific fields or columns are changed
body: <p>I would like to create a column that would store a date only when specific fields/columns are filled with data or have their data changed.</p>
  
  <p>Lets say I have a table named contracts, which has sales person, product sold, date which it was sold etc... Then should someone change the sales person only... Then the date which the sales person was changed should be updated in a specific column. This column should not capture the date when any column is changed, no, just sales person. Been trying to think of the logic to do that but do not know where to start... </p>
  
  <p>Even if I need to use some php its ok please assist... or hints, ideas, suggestion to guide me would be highly appreciated </p>