group_by.result:

  Added  a test case for bug #11385.
group_by.test:
  Added  a test case for bug #11385.
field.h:
  Fixed bug #11385.
  The bug was due to not defined method decimals for the class
  Field_datetime.

mysql-test/r/group_by.result

@@ -732,3 +732,12 @@ SELECT DISTINCT a, b FROM t1 GROUP BY 'const';
 a	b
 1	2
 DROP TABLE t1;
+CREATE TABLE t1 (id INT, dt DATETIME);
+INSERT INTO t1 VALUES ( 1, '2005-05-01 12:30:00' );
+INSERT INTO t1 VALUES ( 1, '2005-05-01 12:30:00' );
+INSERT INTO t1 VALUES ( 1, '2005-05-01 12:30:00' );
+INSERT INTO t1 VALUES ( 1, '2005-05-01 12:30:00' );
+SELECT dt DIV 1 AS f, id FROM t1 GROUP BY f;
+f	id
+20050501123000	1
+DROP TABLE t1;

mysql-test/t/group_by.test

@@ -543,7 +543,7 @@ SELECT hostname, COUNT(DISTINCT user_id) as no FROM t1
 DROP TABLE t1;
 
 #
-# Test for bug #8614: GROUP BY 'const with DISTINCT  
+# Test for bug #8614: GROUP BY 'const' with DISTINCT  
 #
 
 CREATE TABLE t1 (a  int, b int);
@@ -552,3 +552,16 @@ SELECT a, b FROM t1 GROUP BY 'const';
 SELECT DISTINCT a, b FROM t1 GROUP BY 'const';
 
 DROP TABLE t1;
+
+#
+# Test for bug #11385: GROUP BY for datetime converted to decimals  
+#
+
+CREATE TABLE t1 (id INT, dt DATETIME);
+INSERT INTO t1 VALUES ( 1, '2005-05-01 12:30:00' );
+INSERT INTO t1 VALUES ( 1, '2005-05-01 12:30:00' );
+INSERT INTO t1 VALUES ( 1, '2005-05-01 12:30:00' );
+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;

sql/field.h

@@ -25,6 +25,7 @@
 #endif
 
 #define NOT_FIXED_DEC			31
+#define DATETIME_DEC                     6
 
 class Send_field;
 class Protocol;
@@ -861,6 +862,7 @@ public:
   enum ha_base_keytype key_type() const { return HA_KEYTYPE_ULONGLONG; }
 #endif
   enum Item_result cmp_type () const { return INT_RESULT; }
+  uint decimals() const { return DATETIME_DEC; }
   int  store(const char *to,uint length,CHARSET_INFO *charset);
   int  store(double nr);
   int  store(longlong nr);