JOIN KEY 
{ child_key_column | ( child_key_column [, child_key_column]... ) }
REFERENCES parent_level 
[JOIN KEY 
 { child_key_column | ( child_key_column [, child_key_column]... ) }
 REFERENCES parent_level
]...

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