non_join_query_exp

non_join_query_exp 
      ::= 
      non_join_query_term 
      | query_exp "union" [ "all" ] 
      [ corresponding_spec ] query_term 
      | query_exp "except" [ "all" ] 
      [ corresponding_spec ] query_term 

This rule is called by

-> non_join_query_primary
-> query_exp