How do graph databases differ from document databases?

Graph databases and document databases differ primarily in how they structure and query data, each optimized for distinct use cases. Graph databases, like Neo4j or Amazon Neptune, focus on representing relationships between entities. They use nodes (for entities), edges (for relationships), and properties (attributes) to model interconnected data. Document databases, such as MongoDB or Couchbase, store data in flexible, schema-less documents (like JSON), grouping related data into hierarchical structures. The key distinction lies in their handling of relationships: graph databases traverse connections natively, while document databases prioritize self-contained records.

In a graph database, relationships are first-class citizens. For example, in a social network, users (nodes) can be connected via “friendship” edges, and queries efficiently traverse these links to find mutual connections or shortest paths. This is achieved through index-free adjacency, where nodes directly reference their connected edges, avoiding costly joins. Document databases, however, store data in nested documents, which work well for isolated records like user profiles or product catalogs. For instance, an e-commerce product document might include nested arrays for reviews or variants. While document databases can reference other documents (e.g., via IDs), resolving relationships often requires application-level logic or multiple queries, which becomes inefficient for deeply connected data.

The choice between the two depends on the use case. Graph databases excel at scenarios requiring relationship analysis, such as fraud detection, recommendation engines, or network analysis. For example, detecting suspicious financial transactions might involve tracing money flows across accounts. Document databases are better suited for content management, user profiles, or scenarios where data is mostly hierarchical and accessed as a unit. A developer building a blog platform might use a document database to store entire posts with embedded comments and tags. While both can handle some overlapping tasks, graph databases prioritize relationship traversal, while document databases prioritize flexibility and scalability for isolated data structures.