**System Design: Design a Data Warehouse for a Retail Store**\nImagine you are tasked with designing a data warehouse for a retail store chain. The data warehouse should support analytical queries for inventory management, sales reporting, and customer insights. Describe how you would structure the data, what key models you would use, and how you would ensure the system scales with increasing data volume.\n

**LeetCode #15 - 3Sum**\nGiven an array of integers, find all the unique triplets (a, b, c) such that a + b + c = 0, where a, b, and c are distinct indices in the array. You must return the result in a specific order of triplets.\n\n**Example 1:**\nInput: `[-1, 0, 1, 2, -1, -4]`\nOutput: `[[-1, -1, 2], [-1, 0, 1]]`\nExplanation: `-1 + -1 + 2 = 0` and `-1 + 0 + 1 = 0` are the two unique triplets.\n\n**Example 2:**\nInput: `[0, 1, 1]`\nOutput: `[]`\nExplanation: No triplet exists that sums to zero.\n\n**Constraints:**\n- 0 <= nums.length <= 3000\n- -10^5 <= nums[i] <= 10^5\n

**Design a Data Warehouse Schema**\nYou are tasked with designing a data warehouse schema for an online retail company that needs to analyze customer behavior. The company wants to track user purchases, product categories, and sales over time. Describe the overall architecture, including the choice of databases, ETL processes, and data models you would use to achieve this. Discuss how data consistency and scalability can be maintained.\n