LeetCode619. Biggest Single Number

Table: MyNumbers

+-------------+------+
| Column Name | Type |
+-------------+------+
| num         | int  |
+-------------+------+
There is no primary key for this table. It may contain duplicates.
Each row of this table contains an integer.

 A single number is a number that appeared only once in the MyNumbers table.
Write an SQL query to report the largest single number. If there is no single number, report null.
The query result format is in the following example.

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.

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.

SELECT MAX(num) AS num FROM (SELECT num  FROM mynumbers

      GROUP BY num  HAVING COUNT(1) = 1)