RE: [xml-dev] Doubt

["Dare Obasanjo" <dareo@microsoft.com>]

The caveat is that both ID/IDREF and key/keyref only operate on a single XML document not on multiple docs. It looks that this is fine in this case but not a general solution for situations where each table is represented by an XML document.

	-----Original Message----- 
	From: CHIUSANO, Joseph [mailto:JCHIUSANO@lmi.org] 
	Sent: Wed 7/31/2002 1:49 PM 
	To: 'Mudita Singhal'; xml-dev@lists.xml.org 
	Cc: 
	Subject: RE: [xml-dev] Doubt
	
	

	Mudita, 

	You can use the ID/IDREF technique in the DTD - i.e. an attribute of type "ID" would represent the primary key value, and an attribute of type "IDEF" would represent the foreign key value.  The one-to-many relationship would therefore be ID-to-IDREF.  Please note that there are some disadvantages to this approach:

	(1) An ID-type attribute value cannot begin with a number; 
	(2) An ID-type attribute value can appear only once within an XML document (however, IDREF-type attribute values can appear multiple times); 

	(3) An XML processor will not validate the associations between ID and IDREF values.  That is, if A.1 contains the value "A123" and there is a B.2 (IDREF) value of "A123", that is great.  However, if there is a value of "A123" for another IDREF-type attribute (perhaps C.2 instead of B.2), an XML processor will be happy with this even though the association is incorrect.

	To avoid these issues, you can use XML Schema and the KEY/KEYREF mechanism.  In addition to avoiding (1) and (2) above, you can explicitly declare associations between elements or attributes - so you can ensure that (for instance) when there is a value of "A123" for an A.1 attribute, there must be a matching value of "A123" for a B.2 attribute (not just any attribute in the XML document).

	Hope that helps, 
	Joe Chiusano 
	LMI 

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	>   Joseph M. Chiusano 
	>   Logistics Management Institute 
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	-----Original Message----- 
	From: Mudita Singhal [mailto:msinghal3@rediffmail.com] 
	Sent: Wednesday, July 31, 2002 4:12 PM 
	To: xml-dev@lists.xml.org 
	Subject: [xml-dev] Doubt 


	Hello all, 

	I am new to this group and so might be asking a question which 
	might ahve already been answered, or is very simple.Sorry for 
	that. 

	If we have to make a DTD for a database with 2 tables, say A and 
	B.The attributes of the tables are as under: 


	Table A 
	A.1(PRIMARY KEY) 
	A.2 
	A.3(FORIEGN KEY TO TABLE B) 

	Table B 
	B.1(PRIMARY KEY) 
	B.2(foriegn key to table A) 

	Now can anyone guide me on how to write a simple DTD for 
	this.Means , do we have to declare a content type declaration for 
	the forign keys also??IF not then how do we write the XML document 
	for this? 

	secondly How to impose the many-to-many and one-to-many 
	relationships? 

	Thanks 
	Mudita 


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