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.

 

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.

 

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.

MySQL

# 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.

MySQL

# 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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