W3C home > Mailing lists > Public > www-ql@w3.org > July to September 2003

RE: Comparing relationships XML vs. RDBs

From: Paul Cotton <pcotton@microsoft.com>
Date: Thu, 11 Sep 2003 10:54:00 -0400
Message-ID: <E7AC4500EAB7A442ABA7521D1881439706325769@tor-msg-01.northamerica.corp.microsoft.com>
To: <brian@blumenfeld-maso.com>, <www-ql@w3.org>

> This "feels" a lot more SQL-y. But its a completely synthetic example:
I 
> wouldn't keep items and orders all in the same document.

One of the mistakes I think you are making is that you seem to be
assuming that each "document" actually existing physically.  In many
case I think backend stores will provide "virtual" XML views of their
data and permit XQuery on those views.  I call the views "virtual" since
the data will never actually exist physically - the XQueries will simply
be translated directly against the data represented in the views.

If you accept this kind of architecture then you will often have
"documents" like the one you describe.

/paulc

Paul Cotton, Microsoft Canada 
17 Eleanor Drive, Nepean, Ontario K2E 6A3 
Tel: (613) 225-5445 Fax: (425) 936-7329 
mailto:pcotton@microsoft.com

  

> -----Original Message-----
> From: www-ql-request@w3.org [mailto:www-ql-request@w3.org] On Behalf
Of
> Brian Maso
> Sent: September 10, 2003 5:23 PM
> To: www-ql@w3.org
> Subject: Re: Comparing relationships XML vs. RDBs
> 
> 
> (Responding to myself)
> 
> [snip]
> 
>  > Use XML Schemas, there is no standard way to represent a
relationship
> between an element located
>  > in document A to an element in document B. The only intra-element
> relationships are within the
>  > same document. Specifically, the usable relationships are the XPath
> axes
> (child::, parent::,
>  > attribute::, etc.) So intra-entity relationships in XML Schemas
seem a
> bit weaker to me than in
>  > relational schemas.
> 
> Re-reading "Essential XML Quick Reference" (Skonnard, Gudgin) chapter
on
> XML Schema I realize using <xs:key> and <xs:keyref> elements that XML
> Schema supports intra-element references within the same document.
However
> you can't reference between documents, and XPath doesn't have any
support
> for these keyref-based references.
> 
> The following schema describes a relationship between top-level order
> elements and item elements, expressed in individual order-item
elements
> that are children of the order elements.
> 
> <xs:complexType name="itemType">
>    <xs:complexContent>
>      <xs:sequence>
>        <xs:element name="name" type="xs:string"/>
>        ... <!-- description, supplier, etc. --> ...
>      </xs:sequence>
>      <xs:attribute name="id" type="ID" use="required"/>
>    </xs:complexContent>
> </xs:complexType>
> 
> <xs:complexType name="orderType">
>    <xs:complexContent>
>      <xs:sequence>
>        ... <!-- order number, salesperson, etc. --> ...
>        <xs:element name="order-item" type="tns:orderItemType"
>            minOccurs="0" maxOccurs="1"/>
>      </xs:sequence>
>    </xs:complexContent>
> </xs:complexType>
> 
> <xs:complexType name="orderItemType">
>    <xs:complexContent>
>      <xs:sequence>
>        <xs:elemenmt name="quantity" type="xs:nonNegativeInteger"/>
>        ... <!-- color or other options, extra notes, etc. --> ...
>      </xs:sequence>
>      <xs:attribute name="itemID" type="ID"/>
>    </xs:complexContent>
> </xs:complexType>
> 
> <xs:element name="items-and-orders">
>    <xs:complexContent>
>      <xs:sequence>
>        <xs:element name="item" type="itemType"
>                minOccurs="0" maxOccurs="unbounded"/>
>        <xs:element name="order" type="orderType"
>                minOccurs="0" maxOccurs="unbounded"/>
>      </xs:sequence>
>    </xs:complexContent>
> 
>    <xs:key name="keyItemID">
>      <xs:selector xpath="item"/>
>      <xs:field xpath="@id"/>
>    </xs:key>
> 
>    <xs:keyref name="OrderItem_to_Item" refer="keyItemID>
>      <xs:selector xpath="order/order-item"/>
>      <xs:field xpath="@itemID">
>    </xs:keyref>
> </xs:element>
> 
> XQuery main module listing each item name along with the number of
items
> ordered (off the cuff, probably OK -- enough to get the gist):
> 
> for $item in (/items-and-orders/item)
> let $itemID = $item/@id/value(), $itemName=$item/name/value()
> return
>    (
>     $itemName, ": ",
>     fn:sum(/items-and-orders/order/order-item[@itemID =
> $itemID]/quantity/value()),
>     "\n")
>    )
> 
> The relationship between order-item and the item it refers to is
buried in
> the "[@itemID = $itemID]" XPath predicate. (No way to simply use the
> "OrderItem_to_Item" keyref defined in the schema.)
> 
> Using nested FLOWR expressions I guess I could do the same thing like
so:
> 
> for $item in (/items-and-orders/item)
> for $order-item in (/items-and-orders/order/order-item)
> where ($item/@id/value() = $order-item/@itemID/value())
> return
>    (
>     $item/name/value(), ": ",
>     fn:sum($order-item/quantity/value()),
>     "\n")
>    )
> 
> This "feels" a lot more SQL-y. But its a completely synthetic example:
I
> wouldn't keep items and orders all in the same document. What would be
a
> "good" way to define my XML DB? Create a collection of "item"
documents,
> one item per document? Create a single "items" document holding all
"item"
> elements? Create a collection of "order" documents? Or a single
"orders"
> document with multiple "order" elements under it? In any of these
cases I
> wouldn't be able to define the key/keyref combo that defines the
> constraint
> on order-item/@itemID values.
> 
> Any thoughts on how to define the key/keyref with items and orders
stored
> in separate documents?
> 
> Brian Maso
Received on Thursday, 11 September 2003 10:54:23 UTC

This archive was generated by hypermail 2.3.1 : Tuesday, 6 January 2015 20:17:16 UTC