Biggest single number

#
# Table: MyNumbers
# +-------------+------+
# | Column Name | Type |
# +-------------+------+
# | num         | int  |
# +-------------+------+
# This table may contain duplicates (In other words, there is no primary key for this table in SQL).
# Each row of this table contains an integer.
# 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.
# The result format is in the following example.
# Example 1:
# Input:
# MyNumbers table:
# +-----+
# | num |
# +-----+
# | 8   |
# | 8   |
# | 3   |
# | 3   |
# | 1   |
# | 4   |
# | 5   |
# | 6   |
# +-----+
# Output:
# +-----+
# | num |
# +-----+
# | 6   |
# +-----+
# Explanation: The single numbers are 1, 4, 5, and 6.
# Since 6 is the largest single number, we return it.
# Example 2:
# Input:
# MyNumbers table:
# +-----+
# | num |
# +-----+
# | 8   |
# | 8   |
# | 7   |
# | 7   |
# | 3   |
# | 3   |
# | 3   |
# +-----+
# Output:
# +------+
# | num  |
# +------+
# | null |
# +------+
# Explanation: There are no single numbers in the input table so we return null.
#

# Write your MySQL query statement below

SELECT MAX(num) AS num
FROM (
  SELECT num FROM MyNumbers
  GROUP BY num
  HAVING COUNT(*) = 1
) t;