AVG() without group by on non matching row returns NULL

Hi,

I wonder if it's a bug or a feature, but I find this behaviour quite disturbing.
If there's no join and if AVG() is used without GROUP BY and with a WHERE clause on a non matching row, it still returns a result.

e.g. :

MariaDB [(none)]> SELECT AVG(a), b FROM (SELECT 1 as a, 1 as b) a WHERE b=10;

+--------+---+

| AVG(a) | b |

+--------+---+

|   NULL | NULL |

+--------+---+

However, if a GROUP BY is added, no rows are returned :

SELECT AVG(a), b FROM (SELECT 1 as a, 1 as b) a WHERE b=10 GROUP BY a;

Empty set (0.00 sec)

Thanks and regards,
Jocelyn Fournier