title: MySQL update if the person exists
body: <p>what would the MySQL query be to update a record if the record exists in the table i'm trying to update.</p>
  
  <p>so for instance, i'm trying to set a certain column to blank if the record is in the table.  if the record is not in the table, i just don't want it to do anything.</p>
  
  <p>i'm using php and mysql</p>