title: SQL Validation on insert - only allow one or another column to be populated
body: <p>I have a table that allows users to specify a tollerance by an absolute value or by a percentage</p>
  
  <p>e.g. they could say +/- 1,000,000 
  or 10%</p>
  
  <p>so I have a table with a column for each value</p>
  
  <p>What is th best way to ensure that only one or the other gets specified, not both</p>
  
  <p>I will be adding validation to my UI, but would also like this checked by the database</p>
  
  <p>Some kind of custom constraint?</p>