List has no rows for assignment to SObject  error

 Consider following query:

   
      id p;

      user u = [SELECT Id from user where Name = :username];

      if (u != null)

      p = u.Id;

•  The above code will fail if there is no user record with the matching username. It doesn't actually return a null.

 It would be safer to do the following:

    id p;

     user[] userlist = [SELECT Id from user where Name = :username];

     if (userlist.size() > 0)

       p = userlist[0].Id;