Biggest Single Number

Biggest Single Number

What is this problem about?

The "Biggest Single Number interview question" is a SQL aggregation challenge. You are given a table MyNumbers with a single column num. A "single number" is defined as a number that appears only once in the table. Your goal is to find the largest of these single numbers. If no such number exists, the query should return null.

Why is this asked in interviews?

Amazon and Bloomberg ask the "Biggest Single Number coding problem" to test a candidate's knowledge of subqueries and GROUP BY logic. It evaluates whether you can first isolate a subset of data (the single numbers) and then perform a global operation (finding the maximum) on that subset.

Algorithmic pattern used

This problem follows the Subquery with Aggregation pattern.

1. Identify Singles: Use a subquery to select all numbers where the COUNT is exactly 1.
   • SELECT num FROM MyNumbers GROUP BY num HAVING COUNT(num) = 1
2. Find Maximum: Wrap the subquery in another SELECT statement that takes the MAX() of those results.
3. Null Handling: The MAX() function naturally returns null if the input set is empty, satisfying the requirement.

Example explanation

Table: [8, 8, 3, 3, 1, 4, 5, 6]

1. Group and count:
   • 8: Count 2
   • 3: Count 2
   • 1: Count 1
   • 4: Count 1
   • 5: Count 1
   • 6: Count 1
2. Single numbers: {1, 4, 5, 6}.
3. Max: 6. Result: 6.

Common mistakes candidates make

• Missing the HAVING clause: Trying to filter the counts in a WHERE clause, which is invalid for aggregated counts.
• Forgetting the subquery: Trying to use MAX(num) in the same query as GROUP BY, which would return the maximum within each group (the number itself), not the global maximum of the unique numbers.
• Incorrect Output for empty sets: If you don't use the MAX() function correctly, you might return an empty result set instead of a single null value.

Interview preparation tip

Whenever you need to perform an operation on "unique" or "single" items, the GROUP BY ... HAVING COUNT(*) = 1 pattern is your best friend. Master this "Database interview pattern" to solve complex deduplication and reporting tasks.