Key point
One database URL
Your app should read the database connection from an environment variable, usually DATABASE_URL. Do not hard-code the database password in your code.
Local development can use a local database. Production must use the hosted database URL set in your host dashboard.
Pick the right database
Use the same database type your app already uses. If your project uses PostgreSQL locally, choose hosted PostgreSQL. If it uses MySQL, choose hosted MySQL.
Free tiers are enough for most hackathon projects. Neon, Supabase, Render PostgreSQL, Railway, PlanetScale, and MongoDB Atlas are common choices, depending on your database type.