A fix. (Bug #3376: AVG(constant) returns constant if no rows in result set)

mysql-test/r/func_group.result

@@ -256,3 +256,8 @@ bit_and(col)	bit_or(col)
 18446744073709551614	18446744073709551614
 18446744073709551615	18446744073709551615
 drop table t1;
+create table t1 (a int);
+select avg(2) from t1;
+avg(2)
+NULL
+drop table t1;

mysql-test/t/func_group.test

@@ -155,3 +155,11 @@ insert into t1 values (-1), (-2), (-3);
 select bit_and(col), bit_or(col) from t1;
 select SQL_BIG_RESULT bit_and(col), bit_or(col) from t1 group by col;
 drop table t1;
+
+#
+# Bug #3376: avg() and an empty table
+#
+
+create table t1 (a int);
+select avg(2) from t1;
+drop table t1;

sql/item_sum.h

@@ -249,6 +249,7 @@ class Item_sum_avg :public Item_sum_num
   void update_field();
   Item *result_item(Field *field)
   { return new Item_avg_field(this); }
+  void no_rows_in_result() {}
   const char *func_name() const { return "avg"; }
   unsigned int size_of() { return sizeof(*this);}  
 };