SQL Server offers several tools to validate foreign keys, such as built-in features in SSMS like the Database Diagram tool, Object Explorer, and DBCC commands.

Technical documentation for Microsoft SQL Server, tools such as SQL Server Management Studio (SSMS) , SQL Server Data Tools (SSDT) etc. - MicrosoftDocs/sql-docs ...

By using cascading referential integrity constraints, you can define the actions that SQL Server 2005 takes when a user tries to delete or update a key to which existing foreign keys point. Skip ...

There are parent and child entities with foreign key constraints constructed with the option of NO ACTION. The primary key of the parent is known and retrieved from the DB, but the child is not loaded ...

Foreign keys can refer to another column in the same table. The top-level folder might need to be its own parent - I don't know if it'll let you have a NULL there.