类Item_equal

class Item_equal: public Item_bool_func
{
  List<Item_field> fields; /* list of equal field items                    */
  Item *const_item;        /* optional constant item equal to fields items */
  cmp_item *eval_item;
  Arg_comparator cmp;
  bool cond_false;
  bool compare_as_dates;
public:
  inline Item_equal()
    : Item_bool_func(), const_item(), eval_item(), cond_false()
  { const_item_cache= ;}
  Item_equal(Item_field *f1, Item_field *f2);
  Item_equal(Item *c, Item_field *f);
  Item_equal(Item_equal *item_equal);
  inline Item* get_const() { return const_item; }
  void compare_const(Item *c);
  void add(Item *c, Item_field *f);
  void add(Item *c);
  void add(Item_field *f);
  uint members();
  bool contains(Field *field);
  Item_field* get_first() { return fields.head(); }
  void merge(Item_equal *item);
  void update_const();
  enum Functype functype() const { return MULT_EQUAL_FUNC; }
  longlong val_int();
  const char *func_name() const { return "multiple equal"; }
  optimize_type select_optimize() const { return OPTIMIZE_EQUAL; }
  void sort(Item_field_cmpfunc compare, void *arg);
  friend class Item_equal_iterator;
  void fix_length_and_dec();
  bool fix_fields(THD *thd, Item **ref);
  void update_used_tables();
  bool walk(Item_processor processor, bool walk_subquery, uchar *arg);
  Item *transform(Item_transformer transformer, uchar *arg);
  virtual void print(String *str, enum_query_type query_type);
  CHARSET_INFO *compare_collation()
  { return fields.head()->collation.collation; }
};