a fix (Bug #3435: STDDEV|VARIANCE(constant) returns constant if no rows)

mysql-test/r/func_group.result

@@ -648,3 +648,11 @@ select a from t1 having a=1;
 a
 1
 drop table t1;
+create table t1 (a int);
+select variance(2) from t1;
+variance(2)
+NULL
+select stddev(2) from t1;
+stddev(2)
+NULL
+drop table t1;

mysql-test/t/func_group.test

@@ -387,3 +387,12 @@ insert into t1 values (1);
 select max(a) as b from t1 having b=1;
 select a from t1 having a=1;
 drop table t1;
+
+#
+# Bug #3435: variance(const), stddev(const) and an empty table
+#
+
+create table t1 (a int);
+select variance(2) from t1;
+select stddev(2) from t1;
+drop table t1;

sql/item_sum.h

@@ -351,6 +351,7 @@ class Item_sum_variance : public Item_sum_num
   void update_field();
   Item *result_item(Field *field)
   { return new Item_variance_field(this); }
+  void no_rows_in_result() {}
   const char *func_name() const { return "variance"; }
   Item *copy_or_same(THD* thd);
 };