二項演算子だと思って右側に勝手なものを書くとシンタックスエラーになる

MySQLの場合

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mysql> select 2 is 1;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '1' at line 1
mysql> select 2 is not 1;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '1' at line 1
mysql>

PostgreSQLの場合

psql (11.2 (Debian 11.2-1.pgdg90+1))
Type "help" for help.

postgres=# select 2 is 1;
ERROR:  syntax error at or near "1"
LINE 1: select 2 is 1;
                    ^
postgres=# select 2 is not 1;
ERROR:  syntax error at or near "1"
LINE 1: select 2 is not 1;
                        ^
postgres=#

`IS NULL` とか `IS NOT NULL` という単項演算子は存在する

MySQLの場合

mysql> select 2 is null;
+-----------+
| 2 is null |
+-----------+
|         0 |
+-----------+
1 row in set (0.00 sec)

mysql> select 2 is not null;
+---------------+
| 2 is not null |
+---------------+
|             1 |
+---------------+
1 row in set (0.00 sec)

PostgreSQLの場合

postgres=# select 2 is null;
 ?column?
----------
 f
(1 row)

postgres=# select 2 is not null;
 ?column?
----------
 t
(1 row)