help with uniq index

is there a way to create a unique index on a field (in ver 6.3.1 )and also
have a single non-uniq value in that value, ie the NULL value?


Thanks,

     __________________________________________________________________
     *  Lorenzo J. Huerta                                             *
     *  Programming Assistant                                         *
     *  Computing & Networking Networking Architecture and Operations *
     *  New Mexico State University                                   *
     *  phone: (w)(505)646-2582 (h)(505) 521-8699 fax:(505) 646-8139  *
     *  e-mail: lorenzo@nmsu.edu                                      *
     *  Web Site: http://web.nmsu.edu/~ljhuerta                       *
     __________________________________________________________________