Biggest Single Number

Task: A single number is a number that appeared only once in the MyNumbers table. Find the largest single number. If there is no single number, report null.

Table: MyNumbers

+-------------+------+
| Column Name | Type |
+-------------+------+
| num         | INT  |
+-------------+------+

Input: MyNumbers table:

+-----+
| num |
+-----+
| 8   |
| 8   |
| 3   |
| 3   |
| 1   |
| 4   |
| 5   |
| 6   |
+-----+

Output:

+-----+
| num |
+-----+
| 6   |
+-----+