Common Beginner Mistakes

• Reaching for NoSQL because it sounds modern. Most apps have relationships; SQL handles them better.
• Storing money as floating-point. 0.1 + 0.2 is famously not 0.3. Use decimal types.
• No backups. Managed services usually back up automatically — confirm it, don't assume it.
• Putting secrets or huge files in the database. Files belong in object storage (S3, R2); the database stores the link.
• Letting AI write raw queries with user input glued in. This causes SQL injection. Use parameterized queries.
• The N+1 query. A loop that runs one query per item (fetch 100 posts, then 100 separate author lookups) hammers the database. Fetch related data in one query with a join, or batch the lookups.
• Never load-testing. A query that's instant on your laptop's 50 rows can crawl on production's 5 million. Seed a realistic amount of fake data before you trust performance.