Github user chenghao-intel commented on the pull request:
https://github.com/apache/spark/pull/6237#issuecomment-103401627
That's the output from mysql:
```
mysql> create table t4(k int, v int);
Query OK, 0 rows affected (0.17 sec)
mysql> select avg(k) from t4;
+--------+
| avg(k) |
+--------+
| NULL |
+--------+
1 row in set (0.04 sec)
mysql> select avg(k) from t4 group by v;
Empty set (0.00 sec)
mysql>
```
If no group by key specified, it outputs `NULL` otherwise it output empty
set. I think Spark SQL works in the same way.
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