Biggest Single Number - Practice Coding Problems

Biggest Single Number - Problem

Database Easy

Given a database table MyNumbers containing integers, find the largest single number. A single number is defined as a number that appears exactly once in the table.

Table Structure:

MyNumbers
+-------------+------+
| Column Name | Type |
+-------------+------+
| num         | int  |
+-------------+------+

Your Task: Write a SQL query to find the largest number that appears only once. If no such number exists, return null.

Example: If the table contains [8, 8, 3, 3, 1, 4, 5, 6], the single numbers are [1, 4, 5, 6], so the answer is 6.

Input & Output

example_1.sql — Basic Case

$ Input: MyNumbers table: [8, 8, 3, 3, 1, 4, 5, 6]

› Output: 6

💡 Note: Numbers that appear only once: [1, 4, 5, 6]. The largest among them is 6.

example_2.sql — No Singles

$ Input: MyNumbers table: [1, 1, 2, 2, 3, 3]

› Output: null

💡 Note: All numbers appear more than once, so there are no single numbers. Return null.

example_3.sql — All Singles

$ Input: MyNumbers table: [10, 20, 30]

› Output: 30

💡 Note: All numbers appear exactly once: [10, 20, 30]. The largest is 30.

Visualization

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Understanding the Visualization

Group Guests by Room

GROUP BY room_number creates groups of identical room numbers

Filter Single Visits

HAVING COUNT(*) = 1 keeps only rooms visited once

Find Highest Room

MAX(room_number) returns the highest single-visit room

Key Takeaway

🎯 Key Insight: GROUP BY with HAVING is like having a smart hotel manager who can instantly group guests by room and identify single-visit VIPs, all in one efficient operation.

Time & Space Complexity

Time Complexity

⏱️

O(n²)

For each of n distinct numbers, we scan the entire table

⚠ Quadratic Growth

Space Complexity

O(1)

No additional space needed beyond query processing

✓ Linear Space

Constraints

1 ≤ table size ≤ 1000
Numbers can be negative, zero, or positive
-1000 ≤ num ≤ 1000
Table may contain duplicates (no primary key constraint)

Asked in

G Google 45 a Amazon 38 f Meta 32 ⊞ Microsoft 28 Apple 22

The optimal solution uses GROUP BY with HAVING to efficiently group numbers by value, filter groups containing exactly one element, and find the maximum. This approach runs in O(n) time with a single database scan, making it far superior to nested subquery approaches.

Common Approaches

Approach	Time	Space	Notes
✓ Subquery for Each Number	O(n²)	O(1)	For each distinct number, count its occurrences using a subquery
GROUP BY with HAVING (Optimal)	O(n)	O(k)	Group numbers by value, filter groups with count=1, then find maximum

Subquery for Each Number — Algorithm Steps

Get all distinct numbers from the table
For each number, use a correlated subquery to count its occurrences
Filter numbers that appear exactly once (COUNT = 1)
Find the maximum among these single numbers

Visualization

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Step-by-Step Walkthrough

Get Distinct Numbers

Extract all unique numbers from the table

Count Each Number

For each number, run a subquery to count its occurrences

Filter Singles

Keep only numbers that appear exactly once

Find Maximum

Return the largest single number

Code -

solution.c — C

// SQL query as string for database execution
char query[] = "\
SELECT MAX(num) as num \
FROM ( \
    SELECT DISTINCT num \
    FROM MyNumbers m1 \
    WHERE ( \
        SELECT COUNT(*) \
        FROM MyNumbers m2 \
        WHERE m2.num = m1.num \
    ) = 1 \
) single_numbers;";

// Example usage with database library
// sqlite3_exec(db, query, callback, 0, &err);

Time & Space Complexity

Time Complexity

⏱️

O(n²)

For each of n distinct numbers, we scan the entire table

⚠ Quadratic Growth

Space Complexity

O(1)

No additional space needed beyond query processing

✓ Linear Space

Constraints

1 ≤ table size ≤ 1000
Numbers can be negative, zero, or positive
-1000 ≤ num ≤ 1000
Table may contain duplicates (no primary key constraint)

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Input & Output

Visualization

Time & Space Complexity

Related Problems

Constraints

Common Approaches

Subquery for Each Number — Algorithm Steps

Visualization

Code -

Time & Space Complexity

Constraints

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