create table t1 (a int);

create table t2 (b int);

insert into t1 values (3), (7), (1);

insert into t2 values (7), (4), (3);

select * from t1 left join t2 on a=b;

select * from t1 left join t2 on a=b where (b > 3) is not true;

select * from t1 left join t2 on a=b where (b > 3) is not false;

MariaDB [test]> select * from t1 left join t2 on a=b;

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

| a    | b    |

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

|    7 |    7 |

|    3 |    3 |

|    1 | NULL |

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

3 rows in set (0.00 sec)

MariaDB [test]> select * from t1 left join t2 on a=b where (b > 3) is not true;

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

| a    | b    |

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

|    3 |    3 |

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

1 row in set (0.00 sec)

MariaDB [test]> select * from t1 left join t2 on a=b where (b > 3) is not false;

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

| a    | b    |

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

|    7 |    7 |

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

1 row in set (0.00 sec)

postgres=# select * from t1 left join t2 on a=b;

 a | b 

---+---

 1 |  

 3 | 3

 7 | 7

(3 rows)

postgres=# select * from t1 left join t2 on a=b where (b > 1) is not true;

 a | b 

---+---

 1 |  

(1 row)