Explain the differences between a DataFrame and a Series in Pandas. Provide scenarios where each might be preferable. Discuss any underlying implementations, performance considerations, and show how to convert between them if needed.

**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