func_gconcat.result, func_gconcat.test:

  Added test cases for bug #12863.
item_sum.cc, item_sum.h:
  Fixed bug #12863.
  Added a flag to Item_func_group_concat set to FALSE after
  concatenation of the first element of a group.

mysql-test/r/func_gconcat.result

@@ -560,3 +560,23 @@ group_concat('x')
 NULL
 1
 drop table t1;
+CREATE TABLE t1 (id int, a varchar(9));
+INSERT INTO t1 VALUES
+(2, ''), (1, ''), (2, 'x'), (1, 'y'), (3, 'z'), (3, '');
+SELECT GROUP_CONCAT(a) FROM t1;
+GROUP_CONCAT(a)
+,,x,y,z,
+SELECT GROUP_CONCAT(a ORDER BY a) FROM t1;
+GROUP_CONCAT(a ORDER BY a)
+,,,x,y,z
+SELECT GROUP_CONCAT(a) FROM t1 GROUP BY id;
+GROUP_CONCAT(a)
+,y
+,x
+z,
+SELECT GROUP_CONCAT(a ORDER BY a) FROM t1 GROUP BY id;
+GROUP_CONCAT(a ORDER BY a)
+,y
+,x
+,z
+DROP TABLE t1;

mysql-test/t/func_gconcat.test

@@ -356,4 +356,20 @@ select * from (select group_concat(a) from t1) t2;
 select group_concat('x') UNION ALL select 1;
 drop table t1;
 
+#
+# Bug #12863 : missing separators after first empty cancatanated elements
+#
+
+CREATE TABLE t1 (id int, a varchar(9));
+INSERT INTO t1 VALUES
+  (2, ''), (1, ''), (2, 'x'), (1, 'y'), (3, 'z'), (3, '');
+
+SELECT GROUP_CONCAT(a) FROM t1;
+SELECT GROUP_CONCAT(a ORDER BY a) FROM t1;
+
+SELECT GROUP_CONCAT(a) FROM t1 GROUP BY id;
+SELECT GROUP_CONCAT(a ORDER BY a) FROM t1 GROUP BY id;
+
+DROP TABLE t1;
+
 # End of 4.1 tests

sql/item_sum.cc

@@ -1664,7 +1664,9 @@ int dump_leaf_key(byte* key, uint32 count __attribute__((unused)),
   char buff[MAX_FIELD_WIDTH];
   String tmp((char *)&buff,sizeof(buff),default_charset_info), tmp2;
 
-  if (item->result.length())
+  if (item->no_appended)
+    item->no_appended= FALSE;
+  else
     item->result.append(*item->separator);
 
   tmp.length(0);
@@ -1856,6 +1858,7 @@ void Item_func_group_concat::clear()
   result.copy();
   null_value= TRUE;
   warning_for_row= FALSE;
+  no_appended= TRUE;
   if (tree_mode)
     reset_tree(tree);
 }
@@ -1898,8 +1901,7 @@ bool Item_func_group_concat::add()
 
 void Item_func_group_concat::reset_field()
 {
-  if (tree_mode)
-    reset_tree(tree);
+  DBUG_ASSERT(0);
 }
 
 

sql/item_sum.h

@@ -709,6 +709,7 @@ class Item_func_group_concat : public Item_sum
   uint arg_count_field;
   uint field_list_offset;
   uint count_cut_values;
+  bool no_appended;
   /*
     Following is 0 normal object and pointer to original one for copy 
     (to correctly free resources)