This article provides an extensive comparison of four prominent open-source relational databases: PostgreSQL, MySQL, MariaDB, and SQLite. It discusses their architectural differences, advanced data types support, performance, scalability, and development experience. The author emphasizes the importance of understanding core database features to make informed decisions when choosing a suitable database technology for specific use cases. The article concludes by summarizing the strengths and weaknesses of each database and offers guidance on selecting the most appropriate one based on individual needs.