Biggest Single Number

Moderate
0/80
Average time to solve is 20m

Problem statement 

 Table my_numbers contains many numbers in column num including duplicated ones.
 Can you write a SQL query to find the biggest number, which only appears once.

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

For the sample data above, your query should return the following result:
+---+
|num|
+---+
| 6 |

Note: If there is no such number, just output null.

Solution
(100% EXP penalty)
Biggest Single Number
Console