Database choice becomes easier when you start with the shape of your data and the queries your application needs most often.
Relational workloads
PostgreSQL is a strong general-purpose option when you need relational integrity, advanced queries, and extensibility. MySQL remains a practical choice for many web applications and familiar hosting stacks.
Document-oriented workloads
MongoDB can be a good fit when records evolve quickly or your application naturally works with document-shaped data. The important decision is to align the model with the product, not the other way around.