Bug #38637: COUNT DISTINCT prevents NULL testing in HAVING clause

IS NULL was not checking the correct row in a HAVING context.
At the first row of a new group (where the HAVING clause is evaluated)
the column and SELECT list references in the HAVING clause should 
refer to the last row of the previous group and not to the current one. 
This was not done for IS NULL, because it was using Item::is_null() doesn't
have a  Item_is_null_result() counterpart to access the data from the 
last row of the previous group. Note that all the Item::val_xxx() functions 
(e.g. Item::val_int()) have their _result counterparts (e.g. Item::val_int_result()).

Fixed by implementing a is_null_result() (similarly to int_result()) and
calling this instead of is_null() column and SELECT list references inside
the HAVING clause.

mysql-test/r/having.result:
  Bug #38637: test case
mysql-test/t/having.test:
  Bug #38637: test case
sql/item.cc:
  Bug #38637: implement Item::is_null_result() and call it
  from Item_ref and Item_field as appropriate.
sql/item.h:
  Bug #38637: implement Item::is_null_result() and call it
  from Item_ref and Item_field as appropriate.
sql/item_func.cc:
  Bug #38637: implement Item::is_null_result() and call it
  from Item_ref and Item_field as appropriate.
sql/item_func.h:
  Bug #38637: implement Item::is_null_result() and call it
  from Item_ref and Item_field as appropriate.

mysql-test/r/having.result

@@ -424,3 +424,10 @@ select f1 from t1 group by f1 having max(f1)=f1;
 f1
 set session sql_mode='';
 drop table t1;
+CREATE TABLE t1 ( a INT, b INT);
+INSERT INTO t1 VALUES (1, 1), (2,2), (3, NULL);
+SELECT b, COUNT(DISTINCT a) FROM t1 GROUP BY b HAVING b is NULL;
+b	COUNT(DISTINCT a)
+NULL	1
+DROP TABLE t1;
+End of 5.0 tests

mysql-test/t/having.test

@@ -432,3 +432,14 @@ select f1 from t1 having max(f1)=f1;
 select f1 from t1 group by f1 having max(f1)=f1;
 set session sql_mode='';
 drop table t1;
+
+
+#
+# Bug #38637: COUNT DISTINCT prevents NULL testing in HAVING clause
+#
+CREATE TABLE t1 ( a INT, b INT);
+INSERT INTO t1 VALUES (1, 1), (2,2), (3, NULL);
+SELECT b, COUNT(DISTINCT a) FROM t1 GROUP BY b HAVING b is NULL;
+DROP TABLE t1;
+
+--echo End of 5.0 tests

sql/item.cc

@@ -2041,6 +2041,12 @@ bool Item_field::val_bool_result()
 }
 
 
+bool Item_field::is_null_result()
+{
+  return (null_value=result_field->is_null());
+}
+
+
 bool Item_field::eq(const Item *item, bool binary_cmp) const
 {
   Item *real_item= ((Item *) item)->real_item();
@@ -5626,6 +5632,15 @@ double Item_ref::val_result()
 }
 
 
+bool Item_ref::is_null_result()
+{
+  if (result_field)
+    return (null_value=result_field->is_null());
+
+  return is_null();
+}
+
+
 longlong Item_ref::val_int_result()
 {
   if (result_field)
@@ -5731,7 +5746,9 @@ String *Item_ref::val_str(String* tmp)
 bool Item_ref::is_null()
 {
   DBUG_ASSERT(fixed);
-  return (*ref)->is_null();
+  bool tmp=(*ref)->is_null_result();
+  null_value=(*ref)->null_value;
+  return tmp;
 }
 
 

sql/item.h

@@ -652,6 +652,7 @@ class Item {
   virtual my_decimal *val_decimal_result(my_decimal *val)
   { return val_decimal(val); }
   virtual bool val_bool_result() { return val_bool(); }
+  virtual bool is_null_result() { return is_null(); }
 
   /* bit map of tables used by item */
   virtual table_map used_tables() const { return (table_map) 0L; }
@@ -1301,6 +1302,7 @@ class Item_field :public Item_ident
   String *str_result(String* tmp);
   my_decimal *val_decimal_result(my_decimal *);
   bool val_bool_result();
+  bool is_null_result();
   bool send(Protocol *protocol, String *str_arg);
   void reset_field(Field *f);
   bool fix_fields(THD *, Item **);
@@ -1942,6 +1944,7 @@ class Item_ref :public Item_ident
   String *str_result(String* tmp);
   my_decimal *val_decimal_result(my_decimal *);
   bool val_bool_result();
+  bool is_null_result();
   bool send(Protocol *prot, String *tmp);
   void make_field(Send_field *field);
   bool fix_fields(THD *, Item **);

sql/item_func.cc

@@ -4283,6 +4283,15 @@ my_decimal *Item_func_set_user_var::val_decimal_result(my_decimal *val)
 }
 
 
+bool Item_func_set_user_var::is_null_result()
+{
+  DBUG_ASSERT(fixed == 1);
+  check(TRUE);
+  update();					// Store expression
+  return is_null();
+}
+
+
 void Item_func_set_user_var::print(String *str)
 {
   str->append(STRING_WITH_LEN("(@"));

sql/item_func.h

@@ -1299,6 +1299,7 @@ class Item_func_set_user_var :public Item_func
   longlong val_int_result();
   String *str_result(String *str);
   my_decimal *val_decimal_result(my_decimal *);
+  bool is_null_result();
   bool update_hash(void *ptr, uint length, enum Item_result type,
   		   CHARSET_INFO *cs, Derivation dv, bool unsigned_arg);
   bool send(Protocol *protocol, String *str_arg);