SQL문제풀기

[SQL문제풀기] Biggest Single Number

Nov 03, 2025

[SQL문제풀기] Biggest Single Number

Contents

문제

Biggest Single Number - LeetCode

Can you solve this real interview question? Biggest Single Number - 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.

Biggest Single Number - LeetCode

https://leetcode.com/problems/biggest-single-number/description/?envType=study-plan-v2&envId=top-sql-50

Biggest Single Number - LeetCode

MySQL

내가 작성한 정답


select max(num) num
from MyNumbers
where num in (select num
                from MyNumbers
                group by num
                having count(num) =1);

notion image

내가 작성한 오답


select num
from MyNumbers
group by num
having count(num) = 1
order by 1 desc
limit 1;

MyNumbers 테이블에는 단 한 번 등장한 숫자가 없음

→ 그래서 group by + having count(num) = 1 조건을 만족하는 row 자체가 없음

→ 즉 결과 집합이 비어 있음(empty set)

✔ 하지만 빈 결과집합은 NULL을 반환하지 않는다.

→ 그냥 “아무 행도 없음”이기 때문에, MySQL은 결과를 출력하지 않을 뿐

→ NULL을 자동으로 출력해주지 않음

notion image

→ 빈 결과일 때 NULL을 반환하고 싶으면 서브쿼리 사용 + COALESCE 하면 됨.


// 이건 정답
select (
    select num
    from MyNumbers
    group by num
    having count(num) = 1
    order by num desc
    limit 1
) as num;

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