title: Generate Unique hash for a field in SQL Server
body: <p>I'm in the process of writing a Membership Provider for use with our existing membership base.  I use EF4.1 for all of my database access and one of the issued that I'm running into is when the DB was originally setup the relationships were done programmatically instead of in the db.  One if the relationships needs to be made on a column that isn't required for all of our users, but in order to make the relationships does need to be unique (from my understanding).</p>
  
  <p>My solution that I believe will work is to do an MD5 hash on the userid field (which is unique ...which would/should guarantee a unique value in that field).  The part that I'm having issues with on sql server is the query that would do this WITHOUT replacing the existing values stored in the employeeNum field (the one in question).</p>
  
  <p>So in a nutshell my question is.  What is the best way to get a unique value in the <code>employeeNum</code> field (possibly based on an md5 hash of the <code>userid</code> field) on all the rows in which a value isn't already present.  Also, to a minor/major extent...does this sound like a good plan?</p>