{ { simple_expression
  | compound_expression
  | case_expression
  | cursor_expression
  | datetime_expression
  | function_expression
  | interval_expression
  | object_access_expression
  | scalar_subquery_expression
  | type_constructor_expression
  | variable_expression
  }
| expr_list 
}

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