Querying with the native XML datatype
All of our supported databases (Postgres, Oracle, MS SQL) have all had their own native XML datatype for many years now, which allows query writers the ability to run fully fledged XPath queries on them.
EQUELLA doesn’t make use of this functionality though, as an item’s XML is just stored as
a normal text column in the
That doesn’t mean that you can’t use the native XML datatype in your reports however, as you can write SQL that converts the text column to the native XML type and run an XPath query on that.
This tutorial will show how you can retrieve and filter on data contained in the Item
XML using XPath queries, in particular we will be querying the
from our Reporting tutorial schema, filtering our results
based on the number of car’s entered.
Using an XPATH query
In order to return our results, we need to query the Item XML for a count of how many
item/cars/car elements there are.
To query this using with XPath you can use:
NOTE| —| In SQL Server XPath queries will default to being relative to the document, e.g. you still need to reference the <xml> root node.
Let’s put this into our query:
Items with multiple cars
SELECT TOP 10 i.id AS item_id, i.uuid, i.version, i_name.text as item_name, be_name.text as collection_name, ix.car_count FROM item i INNER JOIN (select id, cast (xml as XML).value('count(/xml/item/cars/car)', 'int') as car_count from item_xml) ix on i.item_xml_id = ix.id 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' AND ix.car_count > 1 ORDER BY ix.car_count DESC
Let’s focus on the inner query which is where our xpath query comes into play:
select id, cast (xml as XML).value('count(/xml/item/cars/car)', 'int') as car_count from item_xml
The part that does the XPath query is pretty straight forward:
cast (xml as XML).value('count(/xml/item/cars/car)', 'int')
First we cast the
xml column into the
cast (xml as XML).
Next we call the
value() function on the
XML datatype which takes an XPath query as a string
and a return type for the query. In this case we’re returning the count as an
Once you have the XPath result as an
int type, you can select the id column and
use an INNER JOIN and treat it like any other table:
INNER JOIN (select...) ix on i.item_xml_id = ix.id
We can filter by car_count in the outer query’s where clause to restrict to only those items with multiple cars.
AND ix.car_count > 1
XPath queries are very powerful and they also unlock the power of treating different parts of the Item metadata separately, as you will see in the next tutorial.
Next tutorial : Querying repeated metadata