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:
|On Postgres, XPath queries will default to being relative to the root node, e.g. <xml> for our item xml. If you start your path with a ‘/’ you would need to include the root node too:
Let’s put this into our query:
Items with multiple cars
SELECT i.id AS item_id, i.uuid, i.version,
i_name.text as item_name, be_name.text as collection_name,
FROM item i
INNER JOIN (select id, (xpath('count(item/cars/car)', xml::xml))::text::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, (xpath('count(item/cars/car)', xml::xml))::text::int as car_count from item_xml
The part that does the XPath query is pretty straight forward:
xpath is a function which takes an XPath query as a string and the xml data to run the
xml::xml here because the column in
item_xml is called “xml” and the “
part is the Postgres specific syntax for converting text into the XML datatype.
xmlparse(CONTENT xml) would also have worked instead of
::xml but I have chose the Postgres syntax for brevity.
(...)::text::int part of the query is simply to convert the result of
which is an array of xml, to an integer.
The  part selects the first node from the array, “
::text" converts the xml to
text and “
::int" converts the text number into an integer.
Once you have done the conversion into the
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