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RepeatedMetadata.md

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Querying repeated metadata nodes

In the previous tutorial we learned how to use the value() function to extract data from the item xml.

We did a simply query which returned a single value, however XPath queries are much more powerful than that, they can be used return multiple parts of the xml.

A common pattern within EQUELLA is to collect multiple sets of data using the "Repeater" control, for example:

<xml>
 <item>
  <cars>
   <car>
    <make>Ford</make>
    <model>Laser</model>
   </car>
   <car>
    <make>Hyundai</make>
    <model>Accent</model>
   </car>
   <car>
    <make>Honda</make>
    <model>Accord Euro</model>
   </car>
  </cars>
 </item>
</xml>

What if we'd like to report on each car separately? Thankfully the native XML support of databases can easily handle this problem.

Selecting individual cars

Report on each car

SELECT TOP 10 i.id AS item_id, i.uuid, i.version, 
       i_name.text as item_name, be_name.text as collection_name,
       carxml.query('make/text()') as make,
       carxml.query('model/text()') as model
FROM item i 
INNER JOIN (select id, cast (xml AS XML) itemxml from item_xml) ix on i.item_xml_id = ix.id
CROSS APPLY itemxml.nodes('/xml/item/cars/car') car(carxml)
INNER JOIN base_entity be on be.id = i.item_definition_id
LEFT JOIN language_string i_name on i.name_id = i_name.bundle_id
LEFT JOIN language_string be_name on be.name_id = be_name.bundle_id
WHERE i.institution_id = CURRENT_INSTITUTION
  AND be.uuid = '20d5d5eb-af16-43b0-a42b-962c57fe87f2'

Let's examine the important parts of the query:

INNER JOIN (select id, cast (xml AS XML) itemxml from item_xml) ix on i.item_xml_id = ix.id

This simply converts the converts the textual xml to the native XML datatype and gives it the alias "itemxml".

CROSS APPLY itemxml.nodes('/xml/item/cars/car') car(carxml)

The nodes() XPath function returns a rowset with a single column for each match on the given XPath. In our case we want all the separate cars "/xml/item/cars/car" and we have named the column carxml. CROSS APPLY then combines both rowsets together making a row for each car.

Now that we have rows with individual <car> elements, it's a simple matter of extracting the data from the child nodes:

carxml.query('make/text()') as make,
carxml.query('model/text()') as model

`query()' will return a single result from an XPath query, in our case we're just selecting the text contents.