NULL is treated as 0 in CTE

MariaDB [test]> select version();

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

| version()            |

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

| 10.3.8-MariaDB-debug |

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

1 row in set (0.000 sec)

MariaDB [test]> WITH RECURSIVE qn AS (SELECT 123 AS a UNION ALL

    -> SELECT NULL FROM qn WHERE a IS NOT NULL) SELECT * FROM qn;

+------+

| a    |

+------+

|  123 |

| NULL |

+------+

2 rows in set (0.002 sec)

MariaDB [test]> WITH RECURSIVE qn AS (SELECT 123 AS a UNION ALL

    -> SELECT NULL FROM qn WHERE a IS NOT NULL) SELECT * FROM qn;

^CCtrl-C -- query killed. Continuing normally.

ERROR 1317 (70100): Query execution was interrupted

MariaDB [test]> select version();

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

| version()             |

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

| 10.2.16-MariaDB-debug |

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

1 row in set (0.00 sec)