USE <UserDB>
GO

create table dbo.users ( user_id varchar(20) )
insert into dbo.users select 'test'
GO

USE <WebSiteDB>
GO

CREATE FUNCTION dbo.IsValidUser ( @user_id  varchar(20) )
RETURNS BIT
AS
BEGIN
	DECLARE @Bit BIT
	SET @Bit = 0
	IF EXISTS(SELECT 1 FROM <UserDB>.dbo.users WHERE user_id = @user_id )
		SET @Bit =1
	RETURN @Bit
END
GO

create table websiteDB 
(
user_id varchar(20), CHECK (dbo.IsValidUser(user_id) = 1)
)
GO


--Constraint Catches
Insert websiteDB values ('invalid user')
--OK
Insert websiteDB values ('test')



USE <UserDB>
GO
DROP TABLE users
GO

USE <WebSiteDB>
GO
DROP FUNCTION dbo.IsValidUser
DROP TABLE websiteDB

quote:

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Hopefully in a future version of SQL they will allow for cross database referential integrity.

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quote:

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Remarks
Triggers are often used for enforcing business rules and data integrity. SQL Server provides declarative referential integrity (DRI) through the table creation statements (ALTER TABLE and CREATE TABLE); however, DRI does not provide cross-database referential integrity. To enforce referential integrity (rules about the relationships between the primary and foreign keys of tables), use primary and foreign key constraints (the PRIMARY KEY and FOREIGN KEY keywords of ALTER TABLE and CREATE TABLE). If constraints exist on the trigger table, they are checked after the INSTEAD OF trigger execution and prior to the AFTER trigger execution. If the constraints are violated, the INSTEAD OF trigger actions are rolled back and the AFTER trigger is not executed (fired).

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