What are the differences between relational and hierarchical databases?

Relational and hierarchical databases differ primarily in how they structure and organize data. Relational databases store data in tables with rows and columns, using keys to link related information across tables. For example, a customer’s orders might be stored in a separate table linked to their customer ID. Hierarchical databases, in contrast, organize data in a tree-like structure where each record (node) has a single parent and potentially multiple child nodes. This parent-child model is rigid, resembling an organizational chart where each department branches into specific teams or roles.

A key distinction lies in data access and flexibility. Relational databases use SQL, allowing flexible querying through joins and filters to retrieve data across multiple tables. For instance, combining customer data with their orders and payment history requires joining tables via shared keys. Hierarchical databases, however, require navigating the tree from the root downward. If you want to access a child node, you must traverse through its parent. This can be efficient for specific paths (e.g., fetching all employees under a department) but inflexible for complex relationships. For example, retrieving data from unrelated branches of the hierarchy might involve redundant or slow operations, as hierarchical systems lack built-in mechanisms like joins.

Use cases also differ. Relational databases excel in applications requiring complex transactions and ad-hoc queries, such as e-commerce platforms or banking systems, where data integrity and flexibility are critical. They support ACID compliance and scale horizontally with techniques like sharding. Hierarchical databases are suited for scenarios with inherent hierarchical relationships, like file systems (folders containing files) or legacy systems (e.g., IBM’s IMS). They perform well for read-heavy workloads with predictable access patterns but struggle with many-to-many relationships. For example, adding a new relationship (e.g., tagging products across categories) in a hierarchical system often requires duplicating data or redesigning the structure, whereas relational databases handle this through junction tables. While hierarchical models are less common today, they persist in niche applications where data naturally aligns with a strict hierarchy.