SQL problem - Biggest Single Number

Problem

Query

SELECT IFNULL(
    (SELECT num
    FROM MyNumbers
    GROUP BY num
    HAVING COUNT(num) = 1
    ORDER BY num DESC
    LIMIT 1)
    , NULL) as num

Editorial Solution

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