Most today's database engines do not support foreign keys across databases, servers and engines and that would be very useful to understand data. See how you can document them with Dataedo cross-database relations.
Foreign Keys Articles
See how easily and conveniently you can discover, browse and document table and column relationships in your databases.
Databases should but many don't have foreign key constraints. Does your database have them? To what extent? Here is a method to measure your database.
Results of my research of existence of foreign key constraints in 40 example databases.
My research revealed that most major ERP and CRM applications do not use foreign key constraints in their databases.
A compilation of reasons why databases don't have defined foreign keys.
There are 2 very common traps (with serious consequences) you can easily fall into while joining tables in an SQL query. Learn what they are and how to avoid them.