ORDER [SIBLINGS] BY 
{ expr | position | c_alias } [ ASC | DESC ] [ NULLS FIRST | NULLS LAST ] 
[, { expr | position | c_alias } [ ASC | DESC ] [ NULLS FIRST | NULLS LAST ]]...

For information on the symbols and conventions of Backus-Naur Form (BNF) syntax, see Appendix A.