Skip to main content
Talentd

The Number of Users That Are Eligible for Discount - Solution & Explanation

EasyPremiumFree on FleetCodeDatabase3 min readAsked at: Analytics Quotient
Practice this problem

Problem Statement

Table: Purchases

+-------------+----------+
| Column Name | Type     |
+-------------+----------+
| user_id     | int      |
| time_stamp  | datetime |
| amount      | int      |
+-------------+----------+
(user_id, time_stamp) is the primary key (combination of columns with unique values) for this table.
Each row contains information about the purchase time and the amount paid for the user with ID user_id.

 

A user is eligible for a discount if they had a purchase in the inclusive interval of time [startDate, endDate] with at least minAmount amount. To convert the dates to times, both dates should be considered as the start of the day (i.e., endDate = 2022-03-05 should be considered as the time 2022-03-05 00:00:00).

Write a solution to report the number of users that are eligible for a discount.

The result format is in the following example.

 

Example 1:

Input: 
Purchases table:
+---------+---------------------+--------+
| user_id | time_stamp          | amount |
+---------+---------------------+--------+
| 1       | 2022-04-20 09:03:00 | 4416   |
| 2       | 2022-03-19 19:24:02 | 678    |
| 3       | 2022-03-18 12:03:09 | 4523   |
| 3       | 2022-03-30 09:43:42 | 626    |
+---------+---------------------+--------+
startDate = 2022-03-08, endDate = 2022-03-20, minAmount = 1000
Output: 
+----------+
| user_cnt |
+----------+
| 1        |
+----------+
Explanation:
Out of the three users, only User 3 is eligible for a discount.
 - User 1 had one purchase with at least minAmount amount, but not within the time interval.
 - User 2 had one purchase within the time interval, but with less than minAmount amount.
 - User 3 is the only user who had a purchase that satisfies both conditions.

 

Important Note: This problem is basically the same as The Users That Are Eligible for Discount.

Approach Overview

Problem Overview: The task asks you to count how many users qualify for a discount based on specific eligibility conditions defined in the database tables. You need to filter users that satisfy the rules and return the total number of qualifying users.

Approach 1: Direct Filtering with COUNT (O(n) time, O(1) space)

This problem is solved with a straightforward SQL query. Scan the relevant table and apply the eligibility rules using a WHERE clause. Each row is checked against the required conditions (such as account activity, registration timing, or other attributes defined in the schema). After filtering, use COUNT() to return the total number of users that match the criteria.

The key insight is that SQL engines are optimized for set-based filtering. Instead of iterating manually, you express the eligibility logic declaratively using conditions. The database scans the table once and counts the qualifying rows. If the problem involves multiple tables, a simple JOIN combined with COUNT(DISTINCT ...) ensures each eligible user is counted only once.

This pattern appears frequently in database interview questions. You combine row filtering, optional joins, and aggregation to produce a single numeric result. Understanding when to use COUNT(*) versus COUNT(DISTINCT column) is especially useful when joins can introduce duplicate rows.

The query runs in O(n) time where n is the number of scanned rows. Space complexity is O(1) because the database only maintains an aggregate counter. Concepts used here fall under SQL querying and aggregation.

Recommended for interviews: The direct filtering approach is exactly what interviewers expect. It demonstrates that you can translate business rules into SQL conditions and use aggregation correctly. There is no need for subqueries or complex logic unless duplicates must be removed with DISTINCT.

Solution

Code

MySQL

MySQL

Try this approach in the editor →

Detailed Complexity Analysis

ApproachTimeSpaceWhen to Use
Direct Filtering with COUNTO(n)O(1)Best for most SQL counting problems where rows can be filtered directly using WHERE
JOIN + COUNT(DISTINCT)O(n)O(1)Useful when eligibility depends on data across multiple tables and duplicates must be removed

Video Solution

LeetCode 2205 "Number of Users Eligible for Discount" Interview SQL Question Detailed Explanation • Everyday Data Science • 1,404 views views

Watch 1 more video solutions →

Frequently Asked Questions

Is The Number of Users That Are Eligible for Discount easy or hard?
The problem is categorized as Easy. It mainly tests basic SQL skills such as filtering rows with WHERE and computing aggregates with COUNT. No advanced joins, window functions, or subqueries are typically required.
The Number of Users That Are Eligible for Discount Python/Java solution
This is a database problem, so the expected solution is written in SQL (typically MySQL). Python or Java would only be used to execute the query through a database connector, while the logic itself remains inside the SQL statement.
How to solve The Number of Users That Are Eligible for Discount in O(n)?
Write a SQL query that filters rows using the eligibility conditions in a WHERE clause and returns COUNT(*). The database performs a single table scan, checking each row against the rules and incrementing the aggregate counter.
What is the best approach for The Number of Users That Are Eligible for Discount?
The best approach uses a direct SQL query with a WHERE filter and COUNT aggregation. Apply the eligibility conditions to filter rows and count the remaining users. This solution runs in O(n) time because the database scans the relevant rows once.
Is The Number of Users That Are Eligible for Discount asked at Google/Amazon/Meta?
This problem represents the type of SQL filtering and aggregation questions commonly asked in data engineering and analytics interviews at companies like Amazon, Google, and Meta. Interviewers often test your ability to translate business rules into SQL queries.
What data structure is used in The Number of Users That Are Eligible for Discount?
The solution relies on relational database tables and SQL aggregation. Instead of traditional data structures like arrays or hash maps, the query engine processes rows using filtering conditions and aggregate functions such as COUNT.
What is the time complexity of The Number of Users That Are Eligible for Discount?
The time complexity is O(n), where n is the number of rows scanned by the database. SQL engines evaluate the WHERE conditions for each row and maintain a running COUNT aggregate. Space complexity is O(1) since only the counter is stored.

Ready to solve this problem?

Practice The Number of Users That Are Eligible for Discount with our built-in code editor and test cases.

Practice on FleetCode
Previous

Nth Highest Salary

Next

The Users That Are Eligible for Discount