Both SchemaSpy and SchemaCrawler are free, open-source tools for documenting and analysing relational databases over JDBC. Both have been around for over 20 years. Both can generate entity-relationship diagrams. Yet the two tools are more different than they look.
Disclosure: I work on SchemaCrawler, so take this with appropriate scepticism. I have tried to represent SchemaSpy fairly.
What SchemaSpy Does Best
SchemaSpy's primary strength is its interactive HTML report. After a single run, you get a navigable website: clickable table pages, hyperlinked foreign keys, anomaly reports, and embedded ER diagrams for every table. It is exactly the kind of output you hand to a non-technical stakeholder, a consultant, or a new team member who needs to understand the data model quickly.
SchemaSpy also detects implied relationships - potential foreign keys that are not formally declared in the schema. It provides an orphan table page that surfaces tables with no relationships. These are genuinely useful for legacy databases.
