CREATE TABLE t1 (a int DEFAULT 10);

INSERT INTO t1 VALUES (7), (2), (3);

SELECT * FROM t1 WHERE (SELECT AVG(3)) > 2;

SELECT * FROM t1 WHERE (SELECT AVG(3) OVER ()) > 2;

INSERT INTO t1 VALUES((SELECT avg(4) OVER ()));

SELECT * FROM t1;

MariaDB [test]> CREATE TABLE t1 (a int DEFAULT 10);

Query OK, 0 rows affected (0.017 sec)

MariaDB [test]> INSERT INTO t1 VALUES (7), (2), (3);

Query OK, 3 rows affected (0.002 sec)

Records: 3  Duplicates: 0  Warnings: 0

MariaDB [test]> SELECT * FROM t1 WHERE (SELECT AVG(3)) > 2;

+------+

| a    |

+------+

|    7 |

|    2 |

|    3 |

+------+

3 rows in set (0.003 sec)

MariaDB [test]> SELECT * FROM t1 WHERE (SELECT AVG(3) OVER ()) > 2;

Empty set (0.000 sec)

MariaDB [test]> INSERT INTO t1 VALUES((SELECT avg(4) OVER ()));

Query OK, 1 row affected (0.002 sec)

MariaDB [test]> SELECT * FROM t1;

+------+

| a    |

+------+

|    7 |

|    2 |

|    3 |

| NULL |

+------+

4 rows in set (0.001 sec)