How to handle schema migrations when there’s existing data (with minimal downtime)?

Short answer: Use migrations scripts with data transformation carefully. Apply backward-compatible schema changes first (add columns instead of drop). Use feature flags or phased deployment. Run migrations during maintenance windows or use rolling updates.

Real-world example (ShopNest)

When ShopNest adds a DiscountCode column, you create a migration, review the SQL, then apply it on staging before production.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.