Add Foreign Key relationship between two Databases

You would need to manage the referential constraint across databases using a Trigger.

Basically you create an insert, update trigger to verify the existence of the Key in the Primary key table. If the key does not exist then revert the insert or update and then handle the exception.

Example:

Create Trigger dbo.MyTableTrigger ON dbo.MyTable, After Insert, UpdateAsBegin   If NOT Exists(select PK from OtherDB.dbo.TableName where PK in (Select FK from inserted) BEGIN      -- Handle the Referential Error Here   ENDEND

Edited: Just to clarify. This is not the best approach with enforcing referential integrity. Ideally you would want both tables in the same db but if that is not possible. Then the above is a potential work around for you.

If you need rock solid integrity, have both tables in one database, and use an FK constraint. If your parent table is in another database, nothing prevents anyone from restoring that parent database from an old backup, and then you have orphans.

This is why FK between databases is not supported.

In my experience, the best way to handle this when the primary authoritative source of information for two tables which are related has to be in two separate databases is to sync a copy of the table from the primary location to the secondary location (using T-SQL or SSIS with appropriate error checking - you cannot truncate and repopulate a table while it has a foreign key reference, so there are a few ways to skin the cat on the table updating).

Then add a traditional FK relationship in the second location to the table which is effectively a read-only copy.

You can use a trigger or scheduled job in the primary location to keep the copy updated.