Can you give an example of a situation where tables are related but do not use foreign keys?

Related tables without using foreign keys occur when the relationship is maintained at the application or business logic level, rather than at the database level. For example, in some legacy systems or when working with NoSQL databases, where foreign key support is absent.

Example: there is an orders table and a customers table, where the orders table stores a field called client_id, but no foreign key is set in the database. The relationship is maintained in the code, which performs a join on client_id when fetching data.

This approach can be used to improve performance or simplify migrations, but requires caution to avoid compromising data integrity.