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619. Biggest Single Number

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Description

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.

Solutions

SQL

SELECT MAX(a.num) AS num
FROM
    (
        SELECT num
        FROM MyNumbers
        GROUP BY num
        HAVING count(*) = 1
    ) AS a;