group_by.result, group_by.test:

  Added a test case for bug #11295.
item_buff.cc:
  Fixed bug #11295.
  This a correction for the patch of bug #11088 that takes into
  account a possible NULL values of the BLOB column.

mysql-test/r/group_by.result

@@ -741,3 +741,13 @@ SELECT dt DIV 1 AS f, id FROM t1 GROUP BY f;
 f	id
 20050501123000	1
 DROP TABLE t1;
+CREATE TABLE t1 (id varchar(20) NOT NULL);
+INSERT INTO t1 VALUES ('trans1'), ('trans2');
+CREATE TABLE t2 (id varchar(20) NOT NULL, err_comment blob NOT NULL);
+INSERT INTO t2 VALUES ('trans1', 'a problem');
+SELECT COUNT(DISTINCT(t1.id)), LEFT(err_comment, 256) AS err_comment
+FROM t1 LEFT JOIN t2 ON t1.id=t2.id GROUP BY err_comment;
+COUNT(DISTINCT(t1.id))	err_comment
+1	NULL
+1	a problem
+DROP TABLE t1, t2;

mysql-test/t/group_by.test

@@ -565,3 +565,18 @@ INSERT INTO t1 VALUES ( 1, '2005-05-01 12:30:00' );
 SELECT dt DIV 1 AS f, id FROM t1 GROUP BY f;
 
 DROP TABLE t1;
+
+#
+# Test for bug #11295: GROUP BY a BLOB column with COUNT(DISTINCT column1) 
+#                      when the BLOB column takes NULL values
+# 
+
+CREATE TABLE t1 (id varchar(20) NOT NULL);
+INSERT INTO t1 VALUES ('trans1'), ('trans2');
+CREATE TABLE t2 (id varchar(20) NOT NULL, err_comment blob NOT NULL);
+INSERT INTO t2 VALUES ('trans1', 'a problem');
+
+SELECT COUNT(DISTINCT(t1.id)), LEFT(err_comment, 256) AS err_comment
+  FROM t1 LEFT JOIN t2 ON t1.id=t2.id GROUP BY err_comment;
+
+DROP TABLE t1, t2;

sql/item_buff.cc

@@ -51,8 +51,8 @@ bool Item_str_buff::cmp(void)
   String *res;
   bool tmp;
 
-  res=item->val_str(&tmp_value);
-  res->length(min(res->length(), value.alloced_length()));
+  if ((res=item->val_str(&tmp_value)))
+    res->length(min(res->length(), value.alloced_length()));
   if (null_value != item->null_value)
   {
     if ((null_value= item->null_value))