mysql> create table l (a int);
Query OK, 0 rows affected (0.01 sec)

mysql> create table r (a int);
Query OK, 0 rows affected (0.00 sec)

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

mysql> insert into r values (2),(3),(4);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from l left join r using (a) where r.a is null;
+------+
| a    |
+------+
| 1    |
+------+
1 row in set (0.03 sec)