Practicing “Database Insights”: where to find real schemas?

We talk a lot about “data-driven decisions”, but that usually hides three separate layers: Data itself (events, transactions, logs, etc.). Database structure (schemas, constraints, relationships). Insights on top (from SQL, AI copilots, BI tools, notebooks). My current interest is in that middle layer: using real-world database structures as a playground to practice database insights: Understanding and improving data quality (missing constraints, odd cardinalities, misuse of types