Fixed LP bug #777654.

The method Item_sum_num::fix_fields() calculated the value of
the flag Item_sum_num::maybe_null in some cases incorrectly.

mysql-test/r/sum_distinct.result

@@ -95,3 +95,15 @@ SELECT SUM(DISTINCT id % 11) FROM t1;
 SUM(DISTINCT id % 11)
 55
 DROP TABLE t1;
+#
+# Bug #777654: empty subselect in FROM clause returning  
+#              SUM(DISTINCT) over non-nullable field 
+#
+CREATE TABLE t1 (a int NOT NULL) ;
+SELECT SUM(DISTINCT a) FROM t1;
+SUM(DISTINCT a)
+NULL
+SELECT * FROM (SELECT SUM(DISTINCT a) FROM t1) AS t;
+SUM(DISTINCT a)
+NULL
+DROP TABLE t1;

mysql-test/t/sum_distinct.test

@@ -93,3 +93,15 @@ SELECT SUM(DISTINCT id) FROM t1;
 SELECT SUM(DISTINCT id % 11) FROM t1;
 
 DROP TABLE t1;
+
+--echo #
+--echo # Bug #777654: empty subselect in FROM clause returning  
+--echo #              SUM(DISTINCT) over non-nullable field 
+--echo #
+
+CREATE TABLE t1 (a int NOT NULL) ;
+
+SELECT SUM(DISTINCT a) FROM t1;
+SELECT * FROM (SELECT SUM(DISTINCT a) FROM t1) AS t;
+
+DROP TABLE t1;

sql/item_sum.cc

@@ -568,13 +568,12 @@ Item_sum_num::fix_fields(THD *thd, Item **ref)
     return TRUE;
 
   decimals=0;
-  maybe_null=0;
+  maybe_null= sum_func() != COUNT_FUNC;
   for (uint i=0 ; i < arg_count ; i++)
   {
     if (args[i]->fix_fields(thd, args + i) || args[i]->check_cols(1))
       return TRUE;
     set_if_bigger(decimals, args[i]->decimals);
-    maybe_null |= args[i]->maybe_null;
   }
   result_field=0;
   max_length=float_length(decimals);