The article discusses the differences in syntax and behavior of foreign keys between PostgreSQL and MySQL databases. Foreign keys are crucial for maintaining data integrity and establishing relationships between tables, but their implementation varies between these two database systems. While both support foreign key constraints, PostgreSQL enforces them inline with column definitions, while MySQL ignores them when defined inline and only recognizes them as separate FOREIGN KEY clauses. This can lead to inconsistencies in data if not properly handled during migration from one system to another. The article emphasizes the importance of reviewing schema definitions and testing thoroughly when migrating databases between PostgreSQL and MySQL.