mysql> create table First_Table
    -> (
    -> Id int
    -> );
Query OK, 0 rows affected (0.88 sec)

mysql> insert into First_Table values(1);
Query OK, 1 row affected (0.68 sec)
mysql> insert into First_Table values(2);
Query OK, 1 row affected (0.29 sec)
mysql> insert into First_Table values(3);
Query OK, 1 row affected (0.20 sec)
mysql> insert into First_Table values(4);
Query OK, 1 row affected (0.20 sec)

mysql> select *from First_Table;

+------+
| Id   |
+------+
|   1  |
|   2  |
|   3  |
|   4  |
+------+
4 rows in set (0.00 sec)

mysql> create table Second_Table
    -> (
    -> Id int
    -> );
Query OK, 0 rows affected (0.60 sec)

mysql> insert into Second_Table values(2);
Query OK, 1 row affected (0.19 sec)
mysql> insert into Second_Table values(4);
Query OK, 1 row affected (0.20 sec)

mysql> select *from Second_Table;

+------+
| Id   |
+------+
|   2  |
|   4  |
+------+
2 rows in set (0.00 sec)

mysql> SELECT First_Table.Id FROM First_Table
    -> LEFT OUTER JOIN Second_Table ON First_Table.Id = Second_Table.Id
    -> WHERE Second_Table.Id IS NULL;

+------+
| Id   |
+------+
|   1  |
|   3  |
+------+
2 rows in set (0.00 sec)