title: SQL Constraint: date A is before date B -- How?
body: <p>I'm creating an SQL table that needs the fields <em>from=date</em> and <em>to=date</em> but I'd like to make a constraint so that <em>to</em> cannot be before <em>from</em>. My program will check for it but I'd like to learn how to enforce it with SQL. 
  I've written SQL before but never really used constraints and don't know how they work. </p>
  
  <p>So the question is: Using standard SQL, how do I make sure that <em>From</em> is before <em>To</em>?</p>