relational_expr:
        value_expr [NOT] rel_oper value_expr
   |    value_expr [NOT] BETWEEN constant AND constant
   |    value_expr [NOT] IN (constant [, constant]...) 
   |    column_ref  IS [[NOT]] 
   |    string_expr [NOT] NULL LIKE "string"
   |    [NOT] relational_expr
   |    ( conditional_expr )
   |    value_expr [NOT] rel_oper [{ANY | SOME} | ALL] sub_query
   |    value_expr [NOT]IN sub_query
   |    [NOT] EXISTS sub_query