Fix for BUG#11211 "GROUP BY doesn't work correctly"

When the GROUP BY clause contains a column reference that can be resolved to
both an aliased column in the SELECT list, and to a column in the FROM clause,
the group column is resolved to the column in the FROM clause (for ANSI conformance).
However, it may be so that the user's intent is just the other way around, and he/she
gets the query results grouped by a completely different column than expexted.
This patch adds a warning in such cases that tells the user that there is potential
ambiguity in the group column.

sql/sql_select.cc
- Added a warning when a GROUP column is ambiguous due to that there is a
  column reference with the same name both in the SELECT and FROM clauses.
  In this case we resolve to the column in FROM clause and warn the user
  of a possible ambiguity.
- More extensive comments.
- Changed the function to return bool instead of int (as in other places).

mysql-test/t/group_by.test
  Added test for BUG#11211.

mysql-test/r/group_by.result
  Added test for BUG#11211.


mysql-test/r/group_by.result:
  Import patch 11211.diff
mysql-test/t/group_by.test:
  Import patch 11211.diff
sql/sql_select.cc:
  Import patch 11211.diff
BitKeeper/etc/ignore:
  Added ndb/src/dummy.cpp to the ignore list
mysql-test/r/alias.result:
  Added warning for potentially ambiguous column.
mysql-test/r/having.result:
  Added warning for potentially ambiguous column.

sql/sql_select.cc

@@ -11790,11 +11790,11 @@ cp_buffer_from_ref(THD *thd, TABLE_REF *ref)
     ref_pointer_array.
 
   RETURN
-    0 if OK
-    1 if error occurred
+    FALSE if OK
+    TRUE  if error occurred
 */
 
-static int
+static bool
 find_order_in_list(THD *thd, Item **ref_pointer_array, TABLE_LIST *tables,
                    ORDER *order, List<Item> &fields, List<Item> &all_fields,
                    bool is_group_field)
@@ -11811,13 +11811,13 @@ find_order_in_list(THD *thd, Item **ref_pointer_array, TABLE_LIST *tables,
     {
       my_error(ER_BAD_FIELD_ERROR, MYF(0),
                order_item->full_name(), thd->where);
-      return 1;
+      return TRUE;
     }
     order->item= ref_pointer_array + count - 1;
     order->in_field_list= 1;
     order->counter= count;
     order->counter_used= 1;
-    return 0;
+    return FALSE;
   }
   /* Lookup the current GROUP/ORDER field in the SELECT clause. */
   uint counter;
@@ -11825,7 +11825,7 @@ find_order_in_list(THD *thd, Item **ref_pointer_array, TABLE_LIST *tables,
   select_item= find_item_in_list(order_item, fields, &counter,
                                  REPORT_EXCEPT_NOT_FOUND, &unaliased);
   if (!select_item)
-    return 1; /* Some error occured. */
+    return TRUE; /* The item is not unique, or some other error occured. */
 
 
   /* Check whether the resolved field is not ambiguos. */
@@ -11839,7 +11839,7 @@ find_order_in_list(THD *thd, Item **ref_pointer_array, TABLE_LIST *tables,
     */
     if (unaliased && !order_item->fixed && order_item->fix_fields(thd, tables,
                                                                   order->item))
-      return 1;
+      return TRUE;
 
     /* Lookup the current GROUP field in the FROM clause. */
     order_item_type= order_item->type();
@@ -11879,27 +11879,42 @@ find_order_in_list(THD *thd, Item **ref_pointer_array, TABLE_LIST *tables,
     {
       order->item= ref_pointer_array + counter;
       order->in_field_list=1;
-      return 0;
+      return FALSE;
     }
+    else
+      /*
+        There is a field with the same name in the FROM clause. This is the field
+        that will be chosen. In this case we issue a warning so the user knows
+        that the field from the FROM clause overshadows the column reference from
+        the SELECT list.
+      */
+      push_warning_printf(thd, MYSQL_ERROR::WARN_LEVEL_WARN, ER_NON_UNIQ_ERROR,
+                          ER(ER_NON_UNIQ_ERROR), from_field->field_name,
+                          current_thd->where);
   }
 
   order->in_field_list=0;
   /*
+    The call to order_item->fix_fields() means that here we resolve 'order_item'
+    to a column from a table in the list 'tables', or to a column in some outer
+    query. Exactly because of the second case we come to this point even if
+    (select_item == not_found_item), inspite of that fix_fields() calls
+    find_item_in_list() one more time.
+
     We check order_item->fixed because Item_func_group_concat can put
     arguments for which fix_fields already was called.
-
-    'it' reassigned in if condition because fix_field can change it.
   */
   if (!order_item->fixed &&
       (order_item->fix_fields(thd, tables, order->item) ||
        (order_item= *order->item)->check_cols(1) ||
        thd->is_fatal_error))
-    return 1;					// Wrong field 
+    return TRUE; /* Wrong field. */
+
   uint el= all_fields.elements;
-  all_fields.push_front(order_item);		        // Add new field to field list
+  all_fields.push_front(order_item); /* Add new field to field list. */
   ref_pointer_array[el]= order_item;
   order->item= ref_pointer_array + el;
-  return 0;
+  return FALSE;
 }