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619. Biggest Single Number

Description

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.

Β 

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.

Β 

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

Solution 1: Grouping and Subquery

We can first group the MyNumbers table by num and count the number of occurrences of each number. Then, we can use a subquery to find the maximum number among the numbers that appear only once.

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# Write your MySQL query statement below
SELECT MAX(num) AS num
FROM
    (
        SELECT num
        FROM MyNumbers
        GROUP BY 1
        HAVING COUNT(1) = 1
    ) AS t;

Solution 2: Grouping and CASE Expression

Similar to Solution 1, we can first group the MyNumbers table by num and count the number of occurrences of each number. Then, we can use a CASE expression to find the numbers that appear only once, sort them in descending order by number, and take the first one.

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# Write your MySQL query statement below
SELECT
    CASE
        WHEN COUNT(1) = 1 THEN num
        ELSE NULL
    END AS num
FROM MyNumbers
GROUP BY num
ORDER BY 1 DESC
LIMIT 1;

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