Built a complete SQL Server (T-SQL) data warehouse for a fictional multi-channel, multi-currency retailer (synthetic dataset created for demonstration purposes). Delivered: staging layer loaded from 17 CSV files (~397K rows); one transactional, re-runnable ETL stored procedure that cleans duplicates, mixed date formats and inconsistent labels; a star schema with keys and constraints; reporting views with business rules defined once (net revenue in USD, contribution margin); automated data-quality tests; and 17 analytical queries (cohort retention, RFM segmentation, Pareto/ABC, constant-currency growth, carrier failure alert, inventory health, CAC vs platform claims, budget vs actual). Key insight: marketplace shows 51% gross margin but only 24.5% contribution margin after commission, shipping and refunds.