In this comparative study, we examine the security structures of three prominent database types: relational databases, document databases, and cloud databases. Relational databases, the oldest and most widely used, rely on tables to store structured data, employing security measures like user authentication, authorization, and encryption. Document databases, designed for unstructured data, often used in web applications, employ access control lists (ACLs) and role-based access control (RBAC) to manage data access. Cloud databases, hosted by providers like AWS and Azure, offer scalability and flexibility while implementing standard security features and additional measures like network isolation and data encryption at rest. The study comprises three sections: the first detailing the security structure of relational databases, the second discussing document databases, and the third focusing on cloud databases. The study concludes by comparing the strengths and weaknesses of each database type in terms of security and offering recommendations for enhancing their security measures.