Identify Top-Buying Customers Daily with SQL Query
Company: Amazon
Role: Business Intelligence Engineer
Category: Data Manipulation (SQL/Python)
Difficulty: Medium
Interview Round: Onsite
PRIME
+-------------+------------+----------+
| Customer_ID | Start_Date | End_Date |
+-------------+------------+----------+
| 1 | 2025-01-01 | 2025-12-31|
| 2 | 2025-03-15 | 2025-10-15|
+-------------+------------+----------+
ORDERS
+-------------+----------+------------+-----------+--------------+
| Customer_ID | Order_ID | Order_Day | Product_ID| Quantity_Sold|
+-------------+----------+------------+-----------+--------------+
| 1 | 1001 | 2025-04-01 | P123 | 3 |
| 1 | 1002 | 2025-04-01 | P456 | 2 |
| 2 | 1003 | 2025-04-01 | P123 | 6 |
+-------------+----------+------------+-----------+--------------+
##### Scenario
E-commerce platform wants to identify the top-buying customer(s) for every calendar day based on total quantity sold.
##### Question
Write an SQL query that, for each calendar day, returns the Customer_ID with the highest total Quantity_Sold and the corresponding total units. Return all customers in case of a tie.
##### Hint
Aggregate by day and customer, then use RANK() or DENSE_RANK() over daily totals to select rank = 1.
Quick Answer: This question evaluates a candidate's competency in SQL data manipulation, including aggregation over calendar dates, grouping by customer, and handling ties when determining top buyers.