mysql> create table t1 (row_id int unsigned not null auto_increment primary key, value int default null);
Query OK, 0 rows affected (0.02 sec)

mysql> create table t2 like t1;
Query OK, 0 rows affected (0.02 sec)

mysql> insert into t1 (value) values (1), (2), (NULL);
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> insert into t2 (value) values (1), (2), (NULL);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from t1, t2;
+--------+-------+--------+-------+
| row_id | value | row_id | value |
+--------+-------+--------+-------+
|      1 |     1 |      1 |     1 |
|      2 |     2 |      1 |     1 |
|      3 |  NULL |      1 |     1 |
|      1 |     1 |      2 |     2 |
|      2 |     2 |      2 |     2 |
|      3 |  NULL |      2 |     2 |
|      1 |     1 |      3 |  NULL |
|      2 |     2 |      3 |  NULL |
|      3 |  NULL |      3 |  NULL |
+--------+-------+--------+-------+
9 rows in set (0.00 sec)

mysql> select * from t1, t2 where t1.value = t2.value;
+--------+-------+--------+-------+
| row_id | value | row_id | value |
+--------+-------+--------+-------+
|      1 |     1 |      1 |     1 |
|      2 |     2 |      2 |     2 |
+--------+-------+--------+-------+
2 rows in set (0.01 sec)

mysql> select * from t1, t2 where t1.value = t2.value or t1.value is null or t2.value is null;
+--------+-------+--------+-------+
| row_id | value | row_id | value |
+--------+-------+--------+-------+
|      1 |     1 |      1 |     1 |
|      3 |  NULL |      1 |     1 |
|      2 |     2 |      2 |     2 |
|      3 |  NULL |      2 |     2 |
|      1 |     1 |      3 |  NULL |
|      2 |     2 |      3 |  NULL |
|      3 |  NULL |      3 |  NULL |
+--------+-------+--------+-------+
7 rows in set (0.00 sec)

mysql> select * from t1;
+--------+-------+
| row_id | value |
+--------+-------+
|      1 |     1 |
|      2 |     2 |
|      3 |  NULL |
|      4 |  NULL |
+--------+-------+
4 rows in set (0.00 sec)

mysql> select * from t2;
+--------+-------+
| row_id | value |
+--------+-------+
|      1 |     1 |
|      2 |     2 |
|      3 |  NULL |
+--------+-------+
3 rows in set (0.00 sec)

mysql> select * from t1 left join t2 using (value);
+-------+--------+--------+
| value | row_id | row_id |
+-------+--------+--------+
|     1 |      1 |      1 |
|     2 |      2 |      2 |
|  NULL |      3 |   NULL |
|  NULL |      4 |   NULL |
+-------+--------+--------+
4 rows in set (0.00 sec)

mysql> select * from t1 right join t2 using (value);
+-------+--------+--------+
| value | row_id | row_id |
+-------+--------+--------+
|     1 |      1 |      1 |
|     2 |      2 |      2 |
|  NULL |      3 |   NULL |
+-------+--------+--------+
3 rows in set (0.00 sec)