Maximum Transaction Each Day - Video Solutions

Problem Overview: The task is to return the transaction with the highest amount for each day from a transactions table. If multiple transactions occur on the same day, only the one with the maximum amount should appear in the result.

Approach 1: GROUP BY + Join (O(n log n) time, O(n) space)

One straightforward strategy is to compute the maximum transaction amount per day using GROUP BY day. This produces a small table with day and MAX(amount). Join this result back to the original transactions table on both day and amount. The join filters rows so only transactions matching the daily maximum remain. The database engine typically sorts or hashes groups internally, leading to about O(n log n) time with O(n) intermediate storage.

This method works in almost every SQL dialect and is useful when SQL aggregation functions are preferred over analytic features. However, joins can become less readable as queries grow more complex.

Approach 2: Window Function with ROW_NUMBER() (O(n log n) time, O(n) space)

A cleaner solution uses a window function. Partition transactions by day and rank them by amount in descending order:

ROW_NUMBER() OVER (PARTITION BY day ORDER BY amount DESC)

This assigns rank 1 to the largest transaction for each day. After computing the ranking, filter rows where the rank equals 1. The database sorts rows within each partition to compute the ranking, giving O(n log n) time complexity and O(n) space for intermediate results.

Window functions are designed for exactly this type of per-group ranking problem. They avoid extra joins and keep the query compact. Most modern SQL engines including MySQL, PostgreSQL, and SQL Server support this pattern. It is a common technique in database queries and especially in window function problems.

Recommended for interviews: The window function approach using ROW_NUMBER() is the expected solution. It shows strong familiarity with analytic SQL features and keeps the query concise. The GROUP BY + join approach still demonstrates correct reasoning and works in systems without window function support, but ranking with ROW_NUMBER() is the cleaner and more scalable solution.