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XML Query Use Case R, queries 2, 3, 5, 6, 18

From: Michael Dyck <MichaelDyck@home.com>
Date: Fri, 09 Mar 2001 00:20:36 -0800
Message-ID: <3AA89254.FCD3EF33@home.com>
To: www-xml-query-comments@w3.org
XML Query Use Cases
W3C Working Draft 15 February 2001

Use Case R

(I abbreviate "Expected Result" as "ER", and "Solution in XQuery" as "SiX".)

Query 2 (

In the ER, the <item_tuple> for the Broken Bicycle has no <high_bid> element,
but I don't see how the SiX manages to omit it.  Supposedly, if $b is bound
to an empty list, then
    <high_bid> max($b/bid) </high_bid>
also yields an empty list, but to me this seems far-fetched. Instead, I'd
expect it to either raise an error (from applying "max" to an empty list),
or else yield
(although even that is a bit hard to justify).

To achieve the ER, I think you need to replace
    <high_bid> max($b/bid) </high_bid>
    IF empty($b) THEN [] ELSE <high_bid> max($b/bid) </high_bid>

Query 3 (

The ER uses three element names:
that do not occur in the input documents, but the SiX does not reproduce
them.  I think it needs to replace
    <user_name> $u/name/text() </user_name>
and similarly for the other two.

Query 5 (

The ER has <auction_item> where the SiX has <jones_bike>.

The ER has
    <high_bid> 55 </high_bid>
where the SiX will generate 
    <high_bid><bid> 55 </bid></high_bid>

The ER has
    <bidder> Mary Doe </bidder>
where the SiX will generate
    <high_bidder><name> Mary Doe </name></high_bidder>

The ER includes an item for the Broken Bicycle, but the SiX does not
generate it, because there are no appropriate bindings for $buyer and

I think all of these problems are fixed by the following SiX:

       FOR $seller IN document("users.xml")//user_tuple,
           $item   IN document("items.xml")//item_tuple
       WHERE $seller/name = "Tom Jones"
         AND $seller/userid = $item/offered_by
         AND contains($item/description, "Bicycle")
             $item/itemno ,
             $item/description ,
             FOR $buyer   IN document("users.xml")//user_tuple,
                 $highbid IN document("bids.xml")//bid_tuple
             WHERE $item/itemno = $highbid/itemno
               AND $highbid/userid = $buyer/userid
               AND $highbid/bid =
                                    [itemno = $item/itemno]/bid)
                [ <high_bid> $highbid/bid/text() </high_bid> ,
                  <bidder> $buyer/name/text() </bidder> ]
          </auction_item> SORTBY (itemno)

Query 6 (

"For each item whose highest bid is more than twice its reserve price"...

The SiX has
    WHERE $item/reserve_price < 2 * max($b/bid)
which should be
    WHERE 2 * $item/reserve_price < max($b/bid)
or more readably,
    WHERE max($b/bid) > 2 * $item/reserve_price

Query 18 (

The ER has a single <bid_on_item> element for each distinct item that the
user has bid on, but the SiX will generate multiple <bid_on_item> elements
for the same item, one for each bid ($b) that the user has made on the item.
One solution might be
    $i in distinct( ... )

-Michael Dyck
Received on Friday, 9 March 2001 03:24:33 UTC

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