Method compare_collation

sql/item_cmpfunc.h:1488–1489

  bool walk(Item_processor processor, bool walk_subquery, uchar *arg);
  virtual void print(String *str, enum_query_type query_type);
  const CHARSET_INFO *compare_collation() 
  { return fields.head()->collation.collation; }
}; 

class COND_EQUAL: public Sql_alloc