add support for IS [NOT] DISTINCT FROM

[cyberhuman]

The standard SQL is IS [NOT] DISTINCT FROM, and it's supported by at least Postgre and Firebird.

MySQL uses <=> instead of IS NOT DISTINCT FROM: https://dev.mysql.com/doc/refman/8.0/en/comparison-operators.html#operator_equal-to

mysql> select 0 <=> 0, 0 <=> null, null <=> 0, null <=> null;
+---------+------------+------------+---------------+
| 0 <=> 0 | 0 <=> null | null <=> 0 | null <=> null |
+---------+------------+------------+---------------+
|       1 |          0 |          0 |             1 |
+---------+------------+------------+---------------+
1 row in set (0.00 sec)

sqlite uses IS and IS NOT instead of IS NOT DISTINCT FROM and IS DISTINCT FROM, respectively:

sqlite> select 0 is 0, 0 is null, null is 0, null is null
1|0|0|1

[ygrek]

Thanks for extensive description

[cyberhuman]

Thank you :+1: