LeetCode 619. Biggest Single Number

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Problem

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.

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.

Code

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