for $p in Root("/PersonDB/Person")/Person 
order by $p/FirstName  
return $p/FirstName/data() 

  Frank
  Frank
  Frank
  Frank
  Frank
  Frank
  Frank
  Frank
  Frank
  Frank
  George
  George
  George
  George
  George
  George
  George
  George
  George
  George
  Gwendal
  Gwendal
  Gwendal
  Gwendal
  Gwendal
  Gwendal
  Gwendal
  Gwendal
  Gwendal
  Gwendal
  Jim
  Jim
  Jim
  Jim
  Jim
  Jim
  Jim
  Jim
  Jim
  Jim
  John
  John
  John
  John
  John
  John
  John
  John
  John
  John
  Leif
  Leif
  Leif
  Leif
  Leif
  Leif
  Leif
  Leif
  Leif
  Leif
  Michael
  Michael
  Michael
  Michael
  Michael
  Michael
  Michael
  Michael
  Michael
  Michael
  Paul
  Paul
  Paul
  Paul
  Paul
  Paul
  Paul
  Paul
  Paul
  Paul
  Peter
  Peter
  Peter
  Peter
  Peter
  Peter
  Peter
  Peter
  Peter
  Peter
  Russ
  Russ
  Russ
  Russ
  Russ
  Russ
  Russ
  Russ
  Russ
  Russ