Can a constraint check two different tables for the existence of a parent item?

Question

My database has a table Message, and a table MessageReply.
Each MessageReply has a ParentID and a MessageReplyType (Message, or Reply).
When the MessageReplyType is a message, I would like to ensure the MessageReply.ParentID (MessageID) exists in the Message table.
And when the MessageReplyType is a reply, I need to ensure the MessageReply.ParentID (MessageID) exists in the MessageReply table. 
I am doing this through client code, but I would like the database to enforce this to protect the database from well intentioned programmers like me.
Can it be done?
Thank you

Answer 1

There is a problem with your function and the whole idea of using a check constraint and a function to simulate a foreign key constraint is problematic.
The problem with your function is when you do a statement like
Set @ReturnValue = (Select MessageID From MessageReply Where MessageID = @ParentID);
then if that row does not exist in MessageReply, @ReturnValue is set to NULL.  Then you are returning that NULL from the function and using the check constraint to test whether or not that returned NULL is > 0.  Comparing NULL to a value does
not give you TRUE and it does not give you FALSE, instead you get a logical value called UNKNOWN.  Check constraints only reject a row if the resulting value is FALSE.  It allows the Insert or Update if the logical value is either TRUE or UNKNOWN. 
So even if the parent row does not exist, that check constraint will allow the new row to be inserted.
Second, using a function and check constraint to simulate a foreign key constraint always has a problem.  If you go with that (and you have written the function and constraint correctly) and you attempt to insert a row with a bad ParentID so that the
ParentID does not exist in the appropriate table, you will get a check constraint error and the insert will be rejected.  That is good.  However, suppose you successfully insert a row in MessageReply. For example you insert a row in Message with
MessageID = 1 and then insert a row in MessageReply with ParentID = 1 and ReplyType = 1.  That will work fine as it should.  But now suppose someone deletes the row in Message with MessageID = 1.  That delete won't fire the check constraint. 
So it will allow the delete of the row in Message and now you have a row in MessageReply which doesn't have a parent.
If you want a method that will work completely, you could do that by 1) doing all of your updates to those tables thought stored procedure(s) that check the data to be sure it is valid before inserting/updating/deleting, or 2) writing trigger(s) on both
Message and MessageReply to make sure that any changes are valid, or 3) add computed columns to the MessageReply table so that you can write foreign key constraints. 
I would go with method 3.  That would look something like changing your MessageReply table to look something like
Create Table Message(MessageID int Primary Key
-- other columns as needed
Create Table MessageReply(MessageID int Primary Key
,ParentID int Not Null
,MessageReplyType int Not Null Check (MessageReplyType In (1,2))
,ParentMessageID As Case When MessageReplyType = 1 Then ParentID Else Null End Persisted
,ParentMessageReplyID As Case When MessageReplyType = 2 Then ParentID Else Null End Persisted
-- other columns as needed
Alter Table MessageReply
add constraint chk_MessageParentExists Foreign Key(ParentMessageID) References Message;
Alter Table MessageReply
add constraint chk_MessageReplyParentExists Foreign Key(ParentMessageReplyID) References MessageReply;
Tom